Other factors to consider are air temp and density.

As air flows along the pipe it cools and becomes less dense so needs less pipe area for same flow.

But as the air is now cooler and denser it becomes harder to push.

Also shockwaves moving back and forth, harmonics etc etc.

The more you read about exhaust design the sooner you'll come to the realisation that there are a multitude of factors which all affect one another, complex stuff.

True.

But for a high powered turbo car big is best

Yeah can't say I'm to familiar with turbo exhausts except as you say bigger is better.

I know that turbos interfere with the engines exhaust pulses and scavenging abilities and are a restriction themselves .

The only thing I would say is running as bigger pipe from the turbo as far as you can to the rear axle where the clearance issues usually start should help.

Does anybody opt to run the exhaust under the IRS instead of trying to thread a big exhaust thru it?

I did this with a Jag years ago and although it did hang low it never really scrapped because the low point was between the wheels.

By the time you ran a 4" pipe under the control arm on these cars with enough clearance to not hit the arm when it moved it would be on the ground already.

Obviously you would run the pipe directly under the diff where there is no movement.

The other benefit of this is you keep the pipe a fair bit straighter which also helps flow and only run one large muffler at the rear, again this will improve flow.

If I was going for big HP then this is the way I would go.