Not buying it either but I don't have complicated physics formula's so prove my assumptions!! props to Blackie though.
All I know is the bigger the pressure difference across a surface area (in this case the turbine) the easier it is for it too spool. Anything you have behind the turbo is gonna require or induce extra work to be done to push the gasses out the tips.
IMO the shorter the exhaust, the larger diameter (to some extent) and the least amount of turns + restrictions the better.

 

I asked the same question to a shop that dealt mainly with F bodys. They said they tested a 750 rwhp camaro with a 3" and a 4" system and it had no noticeable gain.

 

That excuse sounds like some *** covering to me.
When I fisrt installed my STS base kit I han no tailpipe. After fitting a 3" pipe from the turbo back to the stock exit location I lost a small amount of boost, maybe .5-1psi and got more lag.
It is noticeably faster and more reponsive without the tail pipe restriction. I don't know how they can say a 3" or 4" does not see any noticeable gain. Maybe on a dyno but not in the real world does that hold true.
Once I revamp my whole system I am going to go with a 4" from the turbo back, maybe use a diesel system's piping and a nice 4" through muffler and resonator. Less restriction = better spool and I would think more power.

 

Ok cool, thanks for confirming it for me guys. Looks like the Corsa will be up for sale here shortly.

 

i would start with one of these duramax turbo backs. there cheap and good quality. its what i had on my silverado. http://www.warpspeedperformance.com/gmdiesel.htm

 

I run a corsa as well. I am running 10, 11 pounds of boost on my 408 and I have noticed that the muffler may not be the problem. I have the pacesetters with the offroad pipe and everytime I lay into it the slip fitting at the collector slides open some. I think it is the bends in the y pipe which slows velocity down a lot.