Proper exhaust size depends on what you're doing. If you're only running pump fuel with the turbo, 3" will be fine but turbo cars will almost always want a larger exhaust. If this is a PT76 GT-S which it more than likely is, I'd generally recommend a 4" system if you're going to be leaning on it at all.
The .96 A/R also would be superior than the .81. Backpressure is a turbo's enemy.
Back pressure should be considered but maintaining exhaust gas velocity
with smaller tubing is just as important
again its like you said "depends on hp wanted"
If you you want high top end power a bigger A/R and exhaust would
be better but you will sacrifice low end with more lag
and vice versa for having quick spool but running out of breathing
room at higher boost levels and upper rpms.
According to "Corky Bell" a 3" down-pipe will typically support up
to 700 flywheel hp.
If you really want to know for sure, buy the book "Maximum Boost"
by Corky Bell. He gives all the math formulas to properly calculate
exhaust gas velocities for a given engine and exhaust tube size.