you don't need a bigger exhaust housing to run more boost, the standard t3 is ok for 12-14psi of boost, not great over that though.
a bigger exhaust housing can be used for more peak power, but at a sacrifice of spooling revs.
i'm a few miles from you and run a 1.8zvh (CR 8.0 IIRC), can't remember pistons though.
and i run a cossie 4wd turbo usign cossie housing and modified down pipe, running on megasquirt and at 10psi it's an animal, 14psi it's spinning t1-r's in third on a dieselling gears form a fiesta 1600, my engine builder built a few in the past and ran around 20psi in them, he doens't really do much these days as he's semi retired now.