At what point does a 2.5" exhaust become restrictive

As per title,
At what point does a 2.5" exhaust become restrictive, tying to plan ahead, the car will be running forged CDB (v9 crank, forged rods, cp pistons & acl bearings) 740's & AFP20g, gt sepc headers.
Would the exhaust i have (2.5" downpipe & ninja 2 system) be restrictive for 400/400 & should i change it to a 3" system or should it be okay.

When i was reading up on forrests site and a few others it seems a 3inch all the way from turbo is recomended like so ive planned for it shame my downpipe has be converted to a 2.5 from a 3 grrr

my old typeR had a revo down pipe which went down too 2.5" at the downpipe too centre section

and it done over 400bhp

if I can remember right it was about 450, I was talking to Bob Rawle about this ages ago, but don't quote me on the exact figure, it's defo over 400

all depends on where the 2.5 inch starts, the hotter the gases then larger bore for same flow, hence why 99% of downpipes are 3 inch even if they drop to 2.5 at flange, most 3 inch systems have a rear silencer with an internal 2.5 inch bore, this is still fine for big power as the gases have cooled rite down,

also too large of exhaust can actualy result in a loss, ie a full freeflow 3 inch system on a car running under 400bhp is an overkill and can give a loss in compared too a 2.5 system

Cheers for the advise lads, think i'll save some money && use the system i have to use, as i the 20g should get it to around 400ish, & not looking for anything more as i don't think the box would take much abuse at that level.