The biggest differences with back pressure is not the exhaust but the header design. Like a 4-1 or a 4-2-1.
The 4-2-1 would be best for our cars unless you have a huge turbo and only want highend.
THEN there is the downpipe. Which I haven't even see him mention. So it's still useless.
KA 3" exhaust before/after dyno
That thread has more information regarding the whole 2.25 vs 3 debate...