these guys have yah covered, you are correct in that you are maxing out the 450s around 300 whp, if your goals are only 350 some 550s would suffice, but it wouldnt hurt in the least to use a set of 750s, that way you have room to grow. as long as you have ARP studs i would not worry about bumping that up to 16 psi, heck it could handle 19 psi. but i think a good tune and 16 psi or so will meet your goals. 350 whp is also my goal so we share something i am currently at 300, next weds i get retuned with a bigger turbo and 750s and am hoping to hit that goal. i hope you reach it as well.