Well, I'm going to do all of that, but the turbine is a Standard Skyline Turbo Turbine ground out to fit the GT30. Do you think that could be too small?
I'd adjust those AFR's down low, you'll be suprised how much of a difference it will make.
As for the power, it doesn't die, it just stops making power after 6000rpm.
I'm going to drop that cat at some stage this week and see what the hell is going on. Might be blocked.