i have a dalton clutch kit in my 800. it works quite well but it has been in since new so i can't compare it to stock. with 27" mudlites it pulls 2-3 quad lengths against a stock 800, mostly out of the hole.
Has anyone tried the EPI clutch kit, the guy at the dealer said it works good on some of the atv's. But who knows if the dealers are feeding you the lines just to sell the product.
X2 on the dalton i have one in my 07 800xt and have installed them in 2 other 800's an 08 and 09 and it was immediately noticeable improvement on all 3. Can give you a hand with the install if you need it. They cost about $225 or so and take 30-45min to install.
Dalton gr/yl or tan spring with 6 Renny weights, or 6 Dalton weights. (The Dalton weights empty weigh the same as a Renny weight with a nearly identical profile)
Edit: the 650 helix helps if you have 28" or larger tires, but will slow you down with near stock sized tires.