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I thought it depended on what power output you were looking at. A 2.5" system is OK for about 350Bhp above that you will probably be better with a 3" system.
Also iirc a 2.5" system will give better low down torque on a standard(ish) car.
Wont be going over 350bhp, well not yet , cheers guys
I would certainly go for the open neck design(not the one that looks like the standard one with closed end opposite to the wastegate exit). A 2,5 would be best if you are not aiming north of 330. Of course 2,5' would be the end of the downpipe(at the point where it is linked with the rest of the exhaust system), cause at the neck, the diameter could be more than 3', depending on manufacturer.