Runs Cool
I have a 2017 M6000 153 2.6 with about 600 miles now. I ride with mainly 800s. During icy morning rides in last spring I realized how different cooling is on the 600. They all would have to stop to cool down, throw snow on the cooler, etc. and my temp never went much over 120 F. Of course I still have to have scratches down and go fast enough to throw snow up, but never over heated. Normally it runs around 105 F. Putting around for kid rides it will heat up a little more on hard pack. I think it is just less heat load from the 600 on the same size (800) cooler. I had a 2016 Axys 800 for a while that ran much hotter in the same conditions.
For out west I would definitely recommend the 153. Will it do everything an 800 can do, no, but I have been really impressed by what this sled can do. I think you would run out of track before you ran out of power on a shorter track. But, I will say it is not fast on the trail. Top speed is about 60 mph and anything above 45 feels like you are flogging it. I still wouldn't change anything because it works so well off trail. If trail speed is important, the shorter track would be better. Anyway, I like it for what it is and I bet the chassis updates and new body make it even better. I do wish they still had the 600 snopro. I think the only 600 mountain sled is the base model.