A 10hp difference will be quite noticeable on the big end. That being said, how frequently do you actually use 100% of your HP? What were the mileage and hours on your last machine? If your like (seemingly) most people, the math would work out to your machine having done an average of 10-15mph. That isnt territory where you'd be "missing" that 10hp all that often. Drag racing buddies.. youd miss it.. everywhere else though.. youd likely be just fine.
i have a friend that had a 550, not sure of the year but I’d guess handy to 2010. I rode it a few times, and for the type of trail riding we do, mostly around 15-25 km/h and the more open sections up to 60km/h the bike had plenty of power. In the trails it wasn’t really a noticeable difference from my 700, but you could notice it on the open sections. Not bad though, guess it depends how you want to ride. I’d buy a 550 if it was in good shape and a good deal!
Absolutely love the 2011 550 that I bought a few years ago for the wife. I've wheeled it quite a bit, and its the reason that I ended up going from a sportsman 800 to a grizz 700. More than enough power to cart my big azz aound the trails, and I've worked on the farm with it as well. Planning on some Coop sheaves for both machines just to get a little more low gearing for the slower trail riding that we do.
My 2013 550 Grizzly has adequate power to pull a heavily loaded Bosski enclosed aluminum trailer with 9 days worth of food, camping gear, for northern BC moose and caribou hunts. Across rivers, up steep mountain trails, in muskeg, mud, snow, etc. Also to push snow with a 60" plow. Good rig. Note that it does not get any better fuel economy than my buddy's 2012 700 Grizzly. About the only downside I can see. Was able to find a better deal on it when I bought it as well. Good luck with it.