The bike has some balls with this combo.
IMO...the Yamaha has plenty of power with just an aftermarket exhaust... more then most people can use on a track. With the stock silencer no doubt it's a dog. It makes different power than the KX (which feels more like a tractor)...the YZ motor is much smoother. I have rode a 09 YZ450 with an exhaust, some very mild head work and a HP CP piston. It motored.
So the bike will produce a nice blast of power with the head work and HC piston? That's what I want just not sure if I can get it out. Of this motor. To be honest I'm just trying to change how the bike produces it's power, I want that tractor hard hit, power RIGHT there when you need it feel.