Here is a look at how three dictionaries of Black English slang have handled the term dope. First, from Clarence Major, Dictionary of Afro-American Slang (1970):
Dope: information; at times also used to refer to illegal drugs but mainly in mockery of "square" usage.
From Geneva Smitherman, Black Talk: Words and Phrases from the Hood to the Amen Corner (1994):
DOPE 1) See DEF ["Great; superb; excellent. ... Also boss, mean, cool, hip, terrible, outa sight, monsta, dynamite (older terms); fresh, hype, jammin, slammin, kickin, bumpin, humpin, phat, pumpin, stoopid[,] stupid, vicious, down, dope, on, raw (newer terms)."] 2) Marijuana, crack, or any other illegal drug.
From Clarence Major, Juba to Jive: A Dictionary of African-American Slang (1994):
Dope n. (1980s–1990s) Mainstream American slang users in the thirties used "dope" to refer to food or to information and only occasionally used it in connection with drugs. It was revived in the eighties as a term for illegal drugs and used frequently on the black street culture scene. (F[ield] R[esearch].) S[outhern and] N[orthern] U[se].
Dope n., adj. (1870s–1990s) information; at times also used to refer to illegal drugs but mainly in mockery of "square" usage; by the 1980s it was being used as an adjective, meaning good or outstanding. (H[yman] E[.] G[oldin, Frank O'Leary & Morris Lipsius], D[ictionary of] A[merican] U[nderworld] L[ingo] [(1950)], p. 60; [Harold] W[entworth & Stuart] F[lexner], D[ictionary of] A[merican] S[lang] [(1967)], p. 156.) Examples: "You get the dope on the situation and we'll take it from there." S[outhern and] N[orthern] U[se].
J.E. Lighter, Random House Historical Dictionary of American Slang (1993) offers this entry for the relevant sense of dope as an adjective:
dope adj. Rap Music. excellent; wonderful; superb; very attractive or enjoyable. [First cited occurrences:] 1981 J. Spicer, in Stanley Rap 301: Yo, man, them boys is dope. ... This record is dope. 1988 N.Y. Times (Aug. 29) C 15: Dope...superb, outstanding...That's a dope Porsche. 1988 Spin (Oct.) 47: Dope, adj., the ultimate...fresh incredible. Ibid. 48: Gucci may be good, but fake Gucci is what's really dope. Ibid. 58: This is a dope jam.
The most striking thing about these dictionary discussions is that none of them finds any use of dope as an adjective before 1981. Lighter, in particular, offers extensive coverage of dope as a noun, identifying instances of slang usage that go back almost 200 years and extend across two dozen distinct meanings: "gravy" (first as doup, 1807), "a stupid person" (1851), "an unidentified unwholesome or poisonous liquid" (1872), "grease" (1876), "medicine or medication of any kind" (1877), "any absorbent or adsorbent solid material used in the manufacture of high explosives" (1880), "butter" (1889), "an alcoholic drink" (1889), "specifically, opium or an opium derivative" (1891), "the Baltimore & Ohio railroad" (1893), "a usu. illegal stupefying or stimulating drug" (1898), "a drug addict" (1899), "coffee" (1899), "stuff (in the broadest sense)" (1899), "information about a racehorse's record, condition, etc." (1899), "full, esp. inside, information of any kind" (1902), "a thick sweet syrup" 1904), "flattery; cajolery; foolishness; nonsense" (1906), "any carbonated soft drink" (1914), "a cigarette" (1918), "a drugged state" (1919), "a slow pitch [in baseball]" (1929), "specifically, marijuana or hashish" (1946), "an ice-cream sundae" (1949), and "sight adjustment [in shooting]" (1987).
Interestingly, Jonathon Green, Chambers Dictionary of Slang (2008) identifies an additional meaning of the noun dope that Lighter either omits or interprets very differently:
dope n. ... 8 {1900s–1910s} (US) constr. with the, the suitable or ideal thing.
Regrettably, the dictionary doesn't cite any contemporaneous instances of this usage. I suspect, though, that Green's meaning "the suitable or ideal thing" may apply to the same subset of slang instances that Lighter assigns the meaning "stuff (in the broadest sense)"; the dates given for the two meanings align fairly well, anyway. Green alludes to the "suitable or ideal thing" meaning again in the entry for the adjective dope:
dope adj. {lit[eral] and fig[urative] uses of DOPE [in the sense of "any form of illicit drug"} 1 {1930s+} pertaining to drugs 2 {1980s+} (US black) (also dope-ass) very good, excellent {note DOPE n (8)}
Green's reference in the entry for adjective dope to "the dope" in its 1900s–1910s sense of "the suitable or ideal thing" appropriately calls attention to their having a similarly positive meaning, but Green stops well short of indicating that the adjective arose from a preserved memory of this rather obscure noun sense, which had died out of common usage sixty years earlier. It would be quite a stretch to claim that origin for the adjective dope.
On the other hand, I see no evidence that the adjective dope, in the sense of "excellent or outstanding," owes its existence to any of the informational senses of the noun dope, either. The early definitions of the adjective form of dope emphasize the notion of being admirable or enjoyable, rather than of being insightful or well informed. It's possible that the original meaning of the adjective dope represents a telescoping of the idea "as good as [being on] dope," in the "drug" sense of dope. But absent a direct recorded connection between the adjective dope and any of its many noun dope predecessors, any such conclusion is ultimately speculative.
Side note on Jimmy Spicer's "Dollar Bill, Y'all (Money)"
J.E. Lighter, Random House Historical Dictionary of American Slang (1993) cites Lawrence A. Stanley, Rap: The Lyrics (1992) as the source of the first use of dope in the sense of "excellent," in which Stanley quotes Jimmy Spicer's "Dollar Bill, Ya'll (Money)" (1981) as follows [combined snippets]:
Jimmy Spicer
Money (Dollar Bill, Y’all)
Yo, man, them boys is dope
Word
Scratchmaster T (Oh man), Supertrooper (Word) — the posse is definitely in effect
Word (word), that's a good record, man
Yo, yo, yo, hold up, m — , yo, can I get a dollar, man?
A dollar?
Anyone got a dollar?
Yeah
You want a dollar?
This is Bill, man, I ain't got no change
I'm kinda thirsty I don't got no change
OK, I'll give you a dollar, you want a dollar?
Well let me hear the record at least
You want a dollar?
First the record
Psyche, I'll tell you what — dollar bill, y'all
This is by Jimmy Spicer
This record is dope, this is about cash money
Dollar bill, y'all, check it out
Dollar bill, y'all
Dollar bill, y'all
Dollar dollar dollar dollar dollar bill, y'all
...
And yet it seems that everything up to the first repetition of "Dollar bill, y'all/ Dollar bill y'all" is conversational studio patter—part of the initial presentation of the rap, but not part of the extremely popular recorded version, which was released in 1983 and can be heard on YouTube in two versions—one of 7 minutes, 14 seconds and one of 4 minutes, 31 seconds.
The upshot of this discussion is that dope in the sense of "excellent" didn't reach a mass audience in the early 1980s by appearing in Spicer's extremely popular and influential rap recording—because it didn't appear in that recording. The spread in usage of dope as "excellent" seems to have been slower and more organic, and Spicer seems to have been an early user but not (on this evidence) a crucial promoter. The actual mechanism of the term's popularization remains to be identified.