As someone who follows tech, I have heard over and over that a product is "X on steroids." Now, outside of a few ailments and allergies that are treated with steroids, it is pretty well accepted that steroids have a negative connotation. Why the dichotomy?
Steroids have a negative connotation with respect to sportsmanship precisely because they make you stronger or faster. "X on steroids" comparisons are taking the sense of improvement into an arena where the sportsmanship aspect is irrelevant, so the negative connotation gets left behind.
"X on steroids" has no negative connotations, unless X is a negative to begin with. ("Leona was like the devil on steroids.") The construction is an intensifier, and just means that something is so good it has an unfair advantage. It is bigger, better, faster, stronger ... whatever qualities one would expect in X's domain, but with an additional multiplier.
Compare it to the use of "uber": Ron is an uber-programmer at my company. It's just one more way of hyping a thing.
First, steroids aren't "that" bad, in popular opinion.
Medically, steroids are used to treatment ailments like arthritis. Chemically, steroids are very common, e.g. choloesterol (though that isn't an anabolic steroid).
Possession or sale of anabolic steroids without a presecription in the U.S. is illegal, but a lot of people don't really care. Probably the most unpopular use of steroids is in sports.
The topic is a light enough one to be the subject of jokes.
Second, steroids are effective, or at least perceived so. If I'm thinking about purchasing product X, but someone tells me, "Y is X on steroids", their telling me how effective it is. Usually, legality/ethics doesn't really even relate to whatever the product is. I don't know of any illegal/unethical lawnmowers.
So it's an apt analogy and not outside the bounds of good taste.
Steroids in a certain context do improve performance or theoretically make someone bigger/more masculine, so the use of "X on steroids" is meant to convey the positive meaning you indicate. However, it seems to me to retain something of a negative connotation in that it's an over-exaggerated or artificial improvement. So if something is described as "X on steroids" it may be improved a bit too much or made a bit too large.
As other answers point out, it's the 'beefing up' aspect of steroids that's being alluded to. The negative connotations for competitive sport and long-term health are irrelevant to this usage, and it's perhaps a bit 'anal' to even think of them in what is after all just an idiomatic coinage.
Even if there are no meaningfully positive connotations, you can get this kind of coinage. Mostly we think the expression Fuck Off is entirely negative (well, mostly it is). But to those familiar with the usage, a fuck-off car is in fact highly desirable.