You'll get a ton of opinions on this one.....here's my take so far.
Holz Rear susp relocate bracket, absolutely necessary! Also read up on the Geo Mods for the rear susp. Good theory, seems to work, can't remember sh!t otherwise. Free mods too.
Offset timing key. I went 2deg advanced, against the recomendation of Eric at Vohk. It works for my sled (fuel pressure has alot to do with it I think). It cleared up a low/mid bog.
87 octane on a stock motor, hell that saves you money. Mine don't run as good on 91.
Brace the running boards! Unless you ride like a little girl or only weigh 100#.
Otherwise you'll be straightening/buying new edges.
Had a cut, straightened, flipped, twisted, modded, blah, blah A 20 when I bought it. $200 for a new Nytro take off Maverick (got $150 for the old POS track) is very nice. Has held up well for 400mi now. Very good in the deep. OK on the trail, worth $50!
Ice scratchers, although it does suprisingly well on harder snow w/o them, it saves the hyfax and sometimes is necessary in order to ride without melting the sliders and overheating (on ice).
Rem extra idler wheels. Free, saves a cpl lbs. Hyfax have 1500 mi now and look good still.
Clutching, always helps. Mine's stock, 'cause half the time it pulls a trailer, so it's not exactly hi markin. Although pulling the sno coach w/ 2 kids in it gives it a harder workout than I ever could by muself.
New Can, lose the weight.