After speaking to several turbo suppliers I think 2.5 inch pipe run is fine for T28 turbo's making anything form 280 - 380 bhp and turbo's producing more than 380 bhp could go up to 3 inch pipe on the cold side.
In doing this though you are giving your turbo more space to fill so you will have a little more lag. The restriction for most people though will still always be the 2.5" inlet into the phlenum, so without increasing the size of that to 3" I see little point in going any bigger than 2.5".
The other problem is the sizes of intercoolers some people use, you see some coolers at 4" thick and the size of a bus strapped to peoples cars, because peolpe think bigger is better. Personally I think the way forward for the car putting out around 350+ bhp would be to have a cooler about 2.5" thick the size of these Hybrid coolers. The problem with the 4" thick coolers is that the air attempting to pass through them has lost so much speed it barely passes through it, thus the cooler is not doing it's job effieciantly, so thinner with a wide exposed surface area is whats needed. I think the R33 cooler or thase new Hybrid coolers are a good example to use, something around 2.5" thick would be good.
