The 2003 MXZ revs had an odd chaincase that they only used that year. It was shorter then the old zx one that they put into the 03.5 summits and all later revs. Does it still have the original chaincase? There was clearance issues with big paddles on them. They also came with out extrovert drivers, have these been changed? The 05 summit had 10tooth drivers that, combined with the zx chaincase, gave alot better aproach angle. is it the original rear skid with just rail extensions, I am not sure, but I think the summit skid sits back farther to allow for driver clearance, and a better approach angle.
I have never rode a long tracked MXZ rev, but I have rode around them and found that they didn't do as well as the summit in the deep snow. Nothin wrong with them on hero snow though.
If you get the 05 summit, check the rings on it and change the skis. The stock skis, if they haven't been changed already, are horrible in most conditions.