I haven't heard anything about the VF34 but I've heard good things about the VF30. The 30 is good enough to drop a WRX into the 12s (with mods to compensate: fuel, etc.)

 

the only diffrence in the 30 and the 34 is the 34 is a ball-bearing turbo.

go with the 34 i am.

 

i have driven 3 wrx's, 2 with turbo xs stage 4 kits, one with a vf30 and one with a vf34. they were both tuned on a dyno with a utec, and there was no noticible difference in spool up. i had a txs stage 4 kit with a ported and polished vf30, there was definatly a difference. i was getting boost at 2800 rpms, and hitting 17.9 psi at 3300 rpms.

 

They basically spool around the same time. I would say save your money and get a VF30, as the VF34 is not really worth the extra money.

 

Definitely look into porting, as this will bring down your lag and increase top end capacity. This was the mod that gave me the single best bang for the buck. Boost comes on 200-300 rpm's lower, and the top end pulls all the way to redline instead of torquing out around 4900 like it used to do.