Forgive the off topic suggestion, but have you considered water/meth injection? A Snow kit on your car would significantly drop IATs and make detonation pretty much a non-issue. I'm sure you know all of this already, though.
The 76GTS was making over 20psi on the dyno last time when the wastegate spring decided to not play nice.
If I remember correctly it hit 23psi, MOAR power!
I wouldnt worry about lag at all. My little Precision Turbo 76GTS .96A/R is the little turbo that could. I have been very impressed with it.
T4 base, small 3" DP and still makes silly boost on my 363.
If I had to do all over T6 would be the way to go.