FMF 4.1 is HUGE, at least in the rear section. Headpipe is still smaller than I'd like to see on a big mod motor. Powercore 4 is tiny, like handlebar tube, and great for 50-88 limited builds.
Pro Circuit has about the best quality, good sizing and makes great power on bigger engines.
Taking the packing out is the silliest thing I've heard in a while. Had I not been told yesterday that the PP-USA super mechanic said zero piston clearance is fine, and that if you heat the jug with a torch, .002" clearance shows up, I'd say the un-pack was best of the week. It will be louder, and make less power. Guaranteed.:lol_hitting:
The stock pipe is probably 28 or 32mm, and looks just fine. When you're floating the valves all the way around the track, and ripping the end off the throttle cable, look for more power
Until then, tossing the crap stock Chikuni oval-bore carb for a "genuine" OKO or "genuiner" Keihin 28mm will make getting to the power it has, a lot easier. Grab a worthy air filter and a TB intake manifold, too. The $150-200 on the intake tract will pay off better than a pipe, no doubt. Maybe grab a chain that won't stretch so fast, to let the sprockets live a while, too.