The first problem is the wide front end of the MXZ's, the original Summits way back when with 670's had 36" front ends and they flat worked, and guess what, 36" is back. Huge difference for throwing it around and side hilling in the powder.
Second, look at the side profile of a Summit and an MXZ, specifically the angle of the track coming up from the tip of the rails to the drivers. Summit's much flatter angle than MXZ's. Flatter angle climbs up on the powder easier, MXZ steep angle is a trenching machine in comparison.
I have fixed this when converting my 08 TNT XP to a mtn machine by moving the skid back about 3 to 4". This was easy to do as they started with 120 tracks. So just worked it out a 137 track and shorter extension plates on rails.
Moving the skid back does haves it drawbacks as far as making it a little more difficult to dial in your suspension. Can be done though, mine works great.
Here is a pic of my 08 TNT 600 carb that only weighs 365 LBs with 15 wide 2.25 X 141 X 3" pitch track. Truly goes anywhere with its light weight and my engine porting with 629 big bore in it. If I was given an 800 if I'd sell it and build another 600 with a Carbon fiber tunnel and get it down in the 340 lb range. Nothing compares to a truly light sled in powder.
Good Luck