I just realized your are the topic originator. Sounds like you are still trying to decide. So I will throw in some more observations so you can see if they match yours.
For the $2,000 difference here is the advantage the M8200 has over the 
M6800 that I see:
 - 15 gross, 11 PTO more HP from turbo
 - CAT II hitch only, but far as I know you can use still use the adapters
 - 1,000 lbs lift capacity
 - Increased fuel capacity 5 gal ROPS, 4 gal cab
 - Increased cooling system
 - approx 1 ft in length
 - about 4" wider
 - 1.5" ground clearance
 - 8" in wheelbase
 - 500 - 1000 lbs in weight depending on model
 - Bigger tires, I think this may be the weight and size difference
Depending on what you will use it for will determine the value of the differences.
So far with my M9000 I have never run out of HP. I always run out of weight first. So I probably would not notice with my current task if I had 15 less HP. I am sure there are implements that would tap the HP. I am sure hydraulics would be one. These tractors have a lot of power.