There is for sure a difference in torque. With the low gearing on the Rmks I honestly don't feel much of a power difference when I jumped on 13 pro 600 to my 8. The six did feel "quicker" down low in rpm.
On older generation polaris, my family has always had a mix of six and 800s. On shorter track sleds (121, 136, 144) you notice more of a difference because of the taller gearing.
Also for what it's worth the 600 Polaris is a bulletproof motor. The odd, and I stress odd, motor will go. However, the majority will run to very high mileage
Personally had:
1998 rmk 600- blew at 13000 miles, carb boot cracked and leaned out pto side, I was young and stupid the
2000 Xc 6000- 11800 when the piston skirt broke and grenades everything
2001 edge x - 8500 miles at trade in, was still running strong
2003 edge x - 6500 miles when sold to a friend, still going. Not sure on mileage
2006 fusion 600 6000 miles when sold to cousin, 8000 miles and still going
2006 rmk 600, crappy water pump gear went at 6500 miles and took out the bottom end
Know of lots of the cfi 2 and cfi 4 motors in my area, have only seen one come through the shop, and in this case maintanence and treatment of the unit were quite suspect looking at overall condition of the unit
Hope this helps
