Seems like a little low for 16.5psi. Did you have the head flowed? You might want to do a little more head work before you do too much on the turbo. Those numbers and rpms tell me it's choking.
Remember, I didn't reach my peak horsepower because of detonation. Once we can spin it up to 5200-5500 rpms we'll probably see 10-25 more horsepower.
Even so, looking at the /6 head compared to a modern head it could use a ton of work. It think the best bang for the buck in the future is going to be even bigger valves and more lift. I need to see how much lift I can get away with since I'm running the slugs about .035" below deck.
I think If I stepped up to a t3/60-1 or even t3/t67 I could get similar power around 10-12psi. The gt32 is pretty small, and needs a lot of pressure to move a lot of air.
