Lots of guys on this site like the FMF powerbomb with the Stroker silencer. White Bro.s E-Series is also popular (and probably the most popular pipe for the YZF). I have the Pro Circuit T4 and like the great bottom and mid and excellent fit, it doesn’t have the chunky header to tailpipe gasket. But pipes are expensive and the YZF isn’t exactly corked up stock so don’t count on too much bang for your buck here. I race off-road and the most important mods for my 426 were: 1) Dez tank. I went opaque IMS and like it fine but people rave about the IMS Ty Davis tank’s narrow feel and the fact that Acerbis tank keeps pet cock on left side. 2) Scott’s damper. Get one and you will not be sorry, the smartest $$ I ever threw at any bike. 3) Ty Davis Bar Mounted Hot Start. Expensive but very trick looking, plus it works great, finding the stock button after a fall in a race was too frustrating/time consuming. 4) Taller seat foam (from Ceet I think, don’t remember) and an N-Style gripper seat cover. The latter is supposed to be a no-no for long races but my butt is either tougher than normal (from days in a real saddle) or the softer foam alleviates the Monkey Butt phenomenon. Either way the two combined to make the bike a lot easier to keep under me, especially when I get tired. I don’t race MX but I imagine some compromise (MX vs. off-road) will be in order when you talk to your suspension tuner. Hope this helps!!