You'll probably get another few psi from up-sizing the turbine discharge. I did some work on a (then new) Riviera factory turbo V6 40 years ago and it was surprising quiet with the entire muffler system disconnected.
The net result from a smaller engine may be difficult to estimate. The smaller number of cylinders tends to make the turbine wheel work a bit harder: more time between the bangs. OTOH, smaller engine makes less exhaust gas and the A/R will be a bit loose.
