In my opinion, "top" is first and foremost an adverb.
top gepflegt, top erhalten
As @Ingmar mentions in his answer, the Duden lists these words as one word but I think this is... nonsensical, Duden or not. Duden is just A dictionary after all.
In some (very rare) occasions the "top" can be fused with the word but "topfit" is the only one I can think of right now, which by the way would be written as two words in English.
"Top" also shows characteristics of an non-declinable adjective.
Das Auto ist in einem top Zustand.
I see no reason why to write that with a hyphen. First of "top" is clearly answering the question "what kind?", which is what adjectives do, but even more important: a sentence like the following (a blue print predicate adjective) is perfectly valid.
Das Bier war top.
I think the hard to pin down nature is indeed due to its English origins.
So... is it a prefix? In my opinion ... no. Can it be used freely? Again, no. It is more flexible as an adjective but as an adverb (superlative-maker) it is really rare. Hard to say when it works and when it doesn't but if it doesn't it will sound really out of place.
Das Auto ist top schnell... what?