Just curious, what alt do you ride at in summer?
I live at about 9k, and only ride high alt aside from an occasional trip to Moab. I ride the same alt in summer & winter... my lunch dirt rides get me to 12k in about 15 min from the driveway, and we ride well into snow season, only difference for winter for me is the track kit.
I'm ok with some mods, probably do a RKT head & a gnarly at least
Summer I stick to less than 2k, unless we chase a race somewhere but even then I don't think we've been above 5-6k. I don't think the gnarly would work good on the snow. You want to run the 300 out and gnarly usually shortens the gear and takes off top end, summer I think its personal preference but I still run a FMF Fatty. I'd look at slavens head and play with the X dimension then let me know what works for next years trip!