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.