Disco, the theory usually never plays out
At leat in my case, I had a 3.9L falcon, with good headers and a 2 1/4" exhaust, changed that to a 3" and it picks up power everywhere, most notably part thottle, was also quieter
Same for the v6 commodore I had, went from 2 1/4 to 2 1/2, again power everywhere
Rx7 was the same
Skyline was the same
My soarer picked up heaps with a dual 2 1/2 in system with crap mufflers over standard
All of the pulled more vacuum at cruise, with a bigger exhaust
From what I understand, no matter the pipe size, the exhaust gas rushed down the pipes at or above Mach 1 hit the static atmosphere at the end of the pipe and bounces back up the pipe to the exhaust ports or a turbo, in the case of our cars
So scavenging in the case of a turbo car is more or less nom existent apart from maybe potentially creating slightly less +ve pressure at the turbine outlet
Also and I'm sure you have probably read it, the scientific design of exhaust and intake systems stats, in basic terms, that any forced induction engine including supercharged motors ruins any chance of tuning your engine via exhaust lengths/size shape etc
Too small will limit power, but the is basically no such thing as too big, in practice, having 2 1/2" primary pipes in a turbo manifold is silly on a streeter rb25, but after the turbo anything goes