I've been debating whether to get a 3" downpipe all along and decided against it. From research, speaking to Barbz and looking at cars Flea have mapped, if you run stock internals and don't want to go over around 350bhp, running the normal downpipe will actually give you more torque.

I'm sure there's people that won't agree with this, but that's what I've found so far.

I cant comment on specific cars here, but, I always use rolling roads to set cars (racecars) up and bigger is not always best. Ive seen people fit 3" exhausts and lose a noticable amount of torque. I remember an article in Track and Racecar mag where they featured a race BMW (old 2002 I think) and it lost 18bhp with a big exhaust, they fitted a pipe about 2ft long in the middle with a narrower bore and got the power back!
Strange but true!!
I would say though, that in general, making the car breath better would make better power, and mapped properly, better torque, although often in a diferent part of the rev range.