I think this 500 vs. 650 thing sounds more like an auto vs. manual thing actually. I'd rather have the manual in the mud.

As far as the tires making a 500 useless, no, not by any means. I had 27" claws on my 99 AC500. Slowed it down a hair on acceleration, but it pulled them just fine in mud. Have to remember, people were using 27" Vampires on Honda 450's, AC & Sportsman 500's all day long a few years back when they were the biggest quads available, and having no problems or real complaints.

A 500 will pull those tires pretty good. Don't know about 28's. You're jumping up the diameter, and I think that hurts your power loss just as much as weight does.

 

I have an 04 Auto 500TBX with 27" Mud Lites and yes there is power loss but not to much that you cant live with. I cant do wheelies or spin out like my buddys Polaris 700 but I go everywhere he does thru some sticky deep mud and I think my bike actually handles it better than his with stock tires on the Polaris. He starts spinning and mine just chugs thru without tire spin. I guess if I was doing over again I would have bought the 650 with the 27s but I love my tbx's bed and they dont offer the 650 with the bed...

 

the reason your buddies belt slipped is cause it is a darn kawasaki setup. other then that kawi crap for a tranny and clutch i know my 650 would walk all over my 500 i had last year in the mud no comparison same drivetrain just the 650 has diff lock and the power. the 500 is a great bike probably the best cat has ever had and if you have a manual then ou can gear yourself before each mudhole to accept the challenge i guess you could say but if you have hte power like i do in my 650 auto is the only way to go, i am always at the edge of my powerband and there is no over revving of the motor.