Well you have to retune your trims anyways, so wether its primary or secondary its the same either way. If you go too big on your primaries than drivability can suffer from side effects, but 850's are proven to be perfect for the street. Stock injectors are 550/850, so its not like the primaries are going to be bigger either, you wouldn't want 1200/850's or anything, but 850's all the way around is a good setup for certain mod setups.

From everything I have read I would keep the 550 primaries and just put bigger scondaries in. Sounds like a fairly good number of people have trouble getting the idle tuned smoothely with 850 primaries. When I finally get around to upgrading my turbo I will probably go with 550 primary and either 1200 or 1600 secondaries, probably 1600. The 1600's work but because they are low impedance and the PowerFC expects high impedance, you must add a resister in-line with the injector.

I have heard that the pfc has a hard time with the 850 primaries unless you have a datalog kit to fine tune it better. With the haltech or something similar to really fine tune 850's work great. Just another reason why i'm glad I ditched my pfc and went haltech, cause at the time 1600's were out of the question and 850's were hard to tune. Now with the use of the datalog kit its posible to use bigger injectors.