There must be something up.
I made 300rwkw with a garrett GT3076 and 0.63 rear housing at 18psi - 20psi. Full boost in third at 3200rpm. That was with the little IW too. You can get a custom garrett IW that is bigger and handles high boost better.
The 0.82 rear housing shits 300rwkw in at 18psi. About 500rpm lagier though.
Its a direct replacement to your current turbo too. Well almost, I think the HKS rear dump pattern is a little different but that is easily fixed.
And why are you wasting money on a 6 boost manifold? All of my results were with a standard manifold. If you want to make the standard flow better (more like a HKS cast one) then hit it with a die grinder and clear out all of the lumps. I did that with the standard manifold I used for my RB25\30 build, and bam, 320rwkw with 20psi from the same Garrett GT3076, 0.82 rear housing, IW. Had Tomei 256\256 poncams though. Full boost in 3rd at 3000rpm, and 820nm of torque at 3000rpm.
Its a weapon. Still going good too.
My advice?
Get a GT3076 or GT3037, 0.82 rear housing.
Port your standard manifold, and get it faced
Port match your Rear housing entrance to your exhaust manifold output
Get some Poncams, 256/256 or 262. Anything bigger is a waste for the RPM range you are using.
Get someone to look at your tune