The lower exhaust pipes are full of screening. not like an RZ350, the air doesnt have to pass thru it, the screens are wrapped around the inside surface of the pipe about 3 times. It's screen like on a screen door. and then it's spot welded into place and retained by these little metal plates. the spot welds are in sets of 3. look at your lower pipes carefully and you will see the sets of 3 spot welds here and there.
I cut open the lower pipes and removed all that crap. and there was plenty of it. grind thru all the welds or maybe there is a better way. I cut the pipe apart near the front cone and also near the rear cone. remove all the retaining metal tabs and that shit, in the baffle (rear) cone the tabs were especially intrusive into the pipe. make the inside of the pipe nice and smooth like it should be!
then reweld. It will be very slightly shorter (width of 2 hacksaw cuts) so no big deal. You will notice the bike sounds much more like a real 2 stroke w/ a good tinny racket coming out the pipes. The screens absorb sound energy, just that simple.
I got 105.5 hp w/ the stock (gutted) pipes. so they are entirely adequate to the task of making very good hp. This was with stock mufflers & all.
beware of muffler swaps or if you do, keep backpressure the same. I put on free-flowing mufflers and lost power. went back to stock.

There is also a ridge of weld inside the stock headpipe. Pull the pipe, look inside, and see the protruding ring of weld? grind that down as flush as you dare.

The upper pipes do not have screens, so you don't have to open them up. Stok pipes provide a nearly flawless powerband and are in tune with the standard exhaust valve settings, as well.