Recently in GermanLanguage.SE, somebody who tried to explain how to pronounce the German vowel "ü" claimed that some speakers of English may use a slightly rounded vowel in Greek loan words like "physical". The dictionaries that I have consulted don't give any alternative pronunciation besides /ɪ/. Are there really native speakers for whom the first vowels in "physical" and "fizzy" are different?
No, the 'y' in Greek loan words like this is invariably just a short 'i' (/ɪ/) sound. In German itself they tend to pronounce these 'y's as ü. This is also the case in Danish, I believe.
"Physical" and "fizzy" have the same vowel phoneme in the first syllable: /ɪ/. Any phonetic difference in quality would be allophonic.
In words from Greek, the letter Y represents the same vowel sounds that I can represent: /ɪ/ (as in sympathy), /aɪ/ (as in phylum), /i/ (as in embryo), /ə/ or /ɜ/ (as in martyr, myrrh). I wrote a more detailed answer talking about the pronunciation of Y as a vowel here: Pronunciation of letter y: asylum vs syrup