There is a little debate between 2.25" and 2.5", but either should be fine. I tend to favor 2.5" (~63.5mm) because it favors future upgrades, be it more aggressive cams or a gentile turbo upgrade or what have you.

 

I don't think it's a myth, it's more like a misunderstanding.

Exhaust is NOT steady-state flow; if it was, then the bigger the better. You always want to allow better exhaust flow, but sometimes a smaller pipe is the way to do just that. It's not back pressure, but it's the exhaust velocity, speed-of-sound, & reflected pulses back into the exhaust ports. They don't cover that stuff in a basic fluid-mechanics course.

 

Im pretty sure 60mm is 2.36", which is what most aftermarket catback piping size's are I think... If you plan on improving air flow IN the engine ie: cams, head work, manifold work, Then the bigger piping would be better, the 2.5", but if you are staying with just i/h/c/e, then 2.36 will be fine I think.