If you do decide to go with the 850 primaries, and you're only looking for 350 hp the 850x4 should be enough with a rewired fuel pump. If you need more fuel later, I say go with the 550/1600. If you shop around you can usually do the swap for about the same as getting the 850's bored out, especially if you sell your rail and 850's. It will be more reliable and give you more fuel. I had a stock 550 fail with no modifications.Replacing a bored out injector "if" it fails would cost more in the end especially if it takes your motor out along with it.

I'm running 550/1680 and it cost me around $350 for the rail, injectors, resistors, clips, and aeromotive FPR, I'm reaching the limits of this setup with my single turbo, and will probably increase the base pressure to get more out of it to run higher boost.

 

I second the 650 primaries.