I am running a OS Speed 21VZ-B V-Spec W.C. w/ a OS 2050 pipe. Major rip and top end. It revs up really fast. A little too much maybe for some or on a smaller track. Still working on a clutch set-up for it.
I have run a Novarossi P5-X before w/ JP3 and a Ninja w/Ninja pipe before that. Both of these I liked, but not as much rip as what I liked.