At what point does a 2.5" exhaust system become limiting ?

Also, does this limiting increase with the power, i.e. it robs a bit at 350hp, but a lot at 380 ?

I've currently fitted a 3"->2.5" sports cat downpipe from H&S. This goes to a 2.5" centre and backbox - the cars a newage, running ~330hp.

I'm considering a TD05 18 or 20g, but have some concerns over the exhaust limitations -

Anyone with any experience of this ?

Ta,

Mark

 

I have been told that the 2.5 inch only starts to become restictive at 350+