I've ridden a 400EX with an FMF Power Core IV muffler. Sweet power gains. More torque. Seems like a different bike. Way too loud. The sound (might) make it feel faster, too.

 

I tried the Powercore IV before on my 400EX and it's almost as loud as the WB E-Series. I am currently using the Supertrapp IDS-2 and it's a lot better in terms of sound (not as loud as the FMF and WB) and power (low to top). Do not forget the Dynojet kit , K&N filters, and airbox vents to really feel the difference.

 

yea a complete system is with the head pipe.