hi guys, another question ref decatting.

I have a 2002 WRX Ej20 T engine - Im gona fit a decat downpipe -
ebay has one which apparently goes from twin port to 2.5 inch. Will the 2.5 inch bore be restrictive or not? what size pipe should i be looking to run to get the best from this system?

IM going to be running a prodrive back box - what size in pipe does this have?
is the prodrive a good exhaust to run?
If anyone has one for sale - back box only (may be interested in other bits too depending on price) let me know.