Steve,
Mike's dead right on that one - it really does depend on where your checkbook is. I'm working at an RCR40 and, initially, I thought I'd go with an Audi 016. I've driven a 40 with the Audi (thanks Hersh!) and I can assure you they are more than sufficient unless you're just damned hard on your equipment and then most likely nothing would be sufficient! Quite a few have the Audi in their 40s and they're working quite fine, thank you very much, even above the 450hp mark.
It's said that the ZF, when treated with respect, will work great right up to 700hp (like you need that much ammo in car that weighs just over 2000lbs). On the other hand, is there such a thing as too much horsepower? Everyone has to answer that one for themselves...
On the off chance you need an Audi, let me know. I have a fresh Jerry Jansing 016 5-speed all ready to go...
Best,
T.