I was using the Goodyear Mudrunner tires on my P650, great tire for any type of terrain & doesn't wear out fast. This tire comes in the 25, 26 or 27 inch sizes. If you do not want to lose too much power, then stay in the stock size or 26 inch.
The most dead weight I have pulled so far has been a Bull Elk, about 1000#'s, my P650 pulled this Elk thru the woods,over dead logs, thru brushes with ease while in 4low, not as heavy as a Moose but I still don't think you would had any problems with a Moose if you had gotten one, as long as you have with Good traction Tires.

 

Curious to why you would switch from p650 to ac 650.

 

That Kawasaki VTwin motor has really spoiled me, with the low end power, gobs of torque & that sweet sound that the VTwin makes. For my major type of riding that I do, I could really used more ground clearance, as on my P650 & even with the 27" tires, I still would hit the skidplates hard on some real nasty trail riding. I have had & owned Arctic Cats in the past years & actually got used to that stupid one brake lever & being the the Cat 650 not only has the Kawie 650 motor but the same transmission with the excellent engine braking, I thought I would give this IRS VTwin & 13 inches of ground clearance a try. I sold my P650 to my brother-in-law, so when I need to do some all-out powersliding down some roads, I can just borrow it for a quick ride.

 

man get some 589s those things are like tank traks the grooves are 2 inches deep or 1 3/4 or somthing.

 

I talked to a guy today who just got back from a moose hunt in Newfoundland. He hauled a moose with his P650; strapped the head to the back rack. When the going was rough he had to get a guy to sit on the front rack to keep the front end down.