standard downpipe size

HI, does anyone know the standard downpipe size, is it 2.25" or 2.5", its hard to measure while on the car with it having that outer skin.
Also would it benefit me if i got a bigger downpipe and still using a cat. I already have a cat back 2.5" straight through.

Thanks in advance

Ash

Some dude on fccuk measure the downpipe and the rest of the exhaust.

Turbo elbow – 2.5 inch ID (Inner Diameter at turbo manifold)
Turbo elbow to Cat Flanged – 2.
Cat inlet pipe – tapers to 2,1/8 inch
Cat – 400csi (only 3rd used by honey comb filament, rest baffler)
Outlet fm Cat – tapers 2 1/7 inch to 2inch

Sleeve fitment Cat back with two rear silencer boxes:
Pipe to mid silencer – 2 inch
Mid Silencer – flat box with S pipe (internal)
Pipe between Mid and rear Silencer – 2 inch
Rear Silencer – entry and exit a same end with u-bend restriction in opposite end (internal).
Tail end comes out of silencer, runs across the top to the rear of the silencer and out to tail pipe – all 2 inch.

Yes you will see some benefit with bigger DP and even more if you change catalysator to a 100 cells cat.

Thank very much