Yes, Deus Ex is OS9 native game, and as i understood there is no OSX release.
Okay, i've tried to do some research. I've tried UT OS9 and UT OSX, compared it with Win laptop (GF2). OS9 release, via classic, gave me 30 fps where windows did 60. But OSX release gave me also 60 fps.
Nope, Classic just got progressively worse before it was dropped. Jaguar was the last time you'd expect a real native-like experience. Much better to just boot into OS 9 when needed.