I use a Mustang dyno. The best hp I got with the 65mm pulley, dual CAI, teflon Y pipe and dyno tune was around 320. My memory is selective so I can't remember if I had the cat delete downpipes or not. 300 hp and 300 tq was the most I got without a good tune though. Your not that far off. If I had to guess it would be that your tune needs adjusting. It sounds like a lean moment when you hit the gas. Did you retune for the manifolds?

Les