Yeah turbovan man bought a wicked wheel for the gm-6, which is what is in all the vans and hummers, at least pre-p400. I know he said the better wheel help a little on the top end, can't remember what else it did off hand. I also would like to find out how else to let one breathe a little better on upper end.
Definitely get a 4" exhaust in there. I've seen that difference in a van and a hummer and I have the parts to do all mine except the muffler.
I spoke to Dennis a couple years back about an ATT, but the 95 and older hummers would have to have a small body lift, and I'm a whined about that. You might talk to him about that in your van, It will pay for itself in fuel savings, then all the power added on is a FREEBIE!
If you do going to a different turbo, PLEASE post it here. I would love to see any other options. I am thinking of educating myself more to learn what improvements I could make to this unit to help it out.
What re you looking for, lower end- upper- or all of it?