Not sure if the extra 17Mhz are a bump-, or clock/bus issue. I'll pass this question to Stachu himself...
Apart from the higher framerates in games, I'd say one of the most noticable speedups are from the WarpUp datatypes, WarpDT. Great software, Amiga needs more of this.
Die hard classic user. Guilty of that, so Im still using 3.9 on both the A3000 and the A4000T. Running W7 if I suddenly feel the urge for semi- transparent jelly windows... 
Oh no, don't say "guilty". I think older Amiga OSes are awesome as long as you know the limitations of what you're doing.
Oh, plus you can run MacOS 9.1 if you have a burning need for anything on that side of the house.
Have you tried PCTask or PCx?