Using an OS plug will void the warranty with your Nova Rossi. OS plugs are not sized well. I wouldn't recommend them anyhow.

The NovaRossi plug Dox recommended is a cold plug. 15% works great! As far as pipe length.. There is a difference in RPM between the NR 50400 and the Mac's muffled pipe (Mac's giving you just a little more). But Jim Oliver has documented this advantage only between 150-200rpm. That is with an 11X7APC. I should say that this is on an older, but new, C61F RE NR.

What pipe and header are you using? There may be an adjustment in the header between the two.

Brian