As far as I know, the f2 thrust pipe doesn't work as an expansion chamber. As in, it doesn't reflect the exhaust gases back into the combustion chamber, and you don't get a big hit of power anywhere in the rpm range. The mz65 does, but you need to be turning enough rpm to get the power out of it.
A real mz65 would just be starting to get into the power band around 6000 to 7000rpm. You would likely see a small bump in rpm up high resulting in a slightly higher top speed, but at the sacrifice of the lower rpm power. Unfortunately no single bolt on pipe on the market is all that well tuned for these motors. The WC80 pipe being the ever so slightly better alternative to the real mz65 due to a ever so slightly longer header tube before the chamber.
A real mz65 would just be starting to get unto the power band around 6000 to 7000rpm. You would likely see a small bump in rpm up high resulting in a slightly higher top speed, but at the sacrifice of the lower rpm power. Unfortunately no single bolt on pipe on the market is all that well tuned for these motors. The WC80 pipe being the ever so slightly better alternative to the real mz65 due to a ever so slightly longer header tube before the chamber.
I have a f2 pipe as well but I run a modified stock exhaust and happier with it. I would like to get more topend, but I'm enjoying my low-end power more. But than again I just put a windowed piston and reed valve on my firestorm 80 and just tuned it again. I don't know what I'll end up doing next