I am still running my stocker, but plan on upgrading to a Yoshi full stainless sysetm soon. The Yoshi system I have on my KFX is very nice. The welds are PERFECT and the piping clears everything just right. I am also very impressed with the sound. I'm not sure about the tri-oval YFZ's stainless system, but the TEC inserts that fit my comp system for the KFX are very nice. Dyno results show almost the same HP with the medium insert vs. no insert, yet db's are around 96 w/ the medium insert. Once I confirm that the new tri-oval system for the YFZ can use the TEC inserts, I'll be buying one.
One word of advice with Yoshi...they have horrible jetting specs on their pipes for the Z400/KFX. They suggest a 150 main with no air box lid and their pipe, but the 150 is too lean. The HP numbers for their pipes would be much higher if they jetted their test bikes better. I don't know what they suggest w/ the YFZ, but it's probably a 165 main at best.