Played around a bit this year with my settup, but never got the time I needed to
really test. Anytime I went up north it was only for a couple of days, and I wanted to ride. Anyway, as z800rtx said, these 800 all have their own personality. I finally settled on thundershift arms, Cudneys primary spring, cudneys secondary spring and helix (which I think is same as stock, only the helix is coated), 27 tooth top gear, rave caps backed out flush with cover, 490 main jets, and MBRP trail silencer. I messed around with a couple other helix's, and secondary springs, and have been back and fourth with the 26 and 27 gear. If I uncouple my rear blocks, and then flip the stoppers up on the rear suspension, I can beat the XC700's and 800's, as well as stock MXZ800's off the line. Another great source of information is on snowest.com, go into the "performance/mod" section and post the question for Big John. He has multiple settups he can give you, and most are just spring, helix, and pin weight changes. One other thing, I am running a Dayco 2 belt, and it was better performing than the stock 127 doo belt, but not as durable. I have a 067 doo belt I will try next, and is a harder compound than the 127. If your friend has the stock exhaust can, look into a restrictor, as some of the guys I have talked to swear by them, and they are cheap.