Well here's my logic...

If the exhaust is meant to bolt up to the catalytic converter, wouldnt the testpipes try to match the length & bolt pattern of the catalytic converter?

The only real choice is 60mm, 63mm or 70mm

 

my other car has a race header that goes straight to the exhaust, eliminating the cat. it's pig rich (mp62 supercharger) and it does leave the soot on the rear bumper (a combination of no cat and low air-fuel ratio)...

 

exhaust tips get it worse than my bumper.. well, it's black though so I couldn't tell anyway.