Hey DAN!!!!
Well i got a Garrett GT30 rated to 600HP, its externally gated and you will definitely need a new manifold!! I just picked up a second hand Trust item, paid a little more than i wanted, but its in really good nick... I'm getting the manifold and exhaust housing ceramic coated...
I've already moved to top feed GTR 440cc injectors, if they are too small then i can get cheap RX7 550cc injectors...
Because the GT30 can flow so much air you will definitely need a bigger AFM, maybe you should ditch the AFM and go MAP, like me!!!
Also from what i've heard, running at the same boost you should see about an extra 75-100rwhp... I'm planning on running no more than .7bar, last thing i wanna do is fold a con rod and have it say hello out the side of the block... I'm also thinking about a metal head gasket... Then i could run a little more boost...