Like CB.8 said, mods are dependent on your budget and what type of riding you do. If I was looking at your sled this is what I'd do in order of importance:
1. Skis. I have Powder Pros for early/mid season and Proclimb 7 G2 off of the new sleds for spring. The G2 bottoms re-use your ski loops and skags. Need new mounting hardware are dampers though. SLP Mohawks ($450 new) and Polaris Grippers ($250 takeoffs) are also popular.
2. Manual chain case tensioner. Can be done with Cat or Yamaha parts.
3. Tube Running boards: $450
- Adds a couple pounds but the added rigidity and transfer of rider input to the sled is well worth it.
- Sheds snow a little better.
- More aggressive grip.
4. Cat magnetic tether.
5. Diamond S Ti Quiet can: $580
- Drops a ton of weight
- Adds HP
- Not loud.
6. Shock Revalve: depends on shop.
7. Hood with intake and headlight delete: $400
- 8-10lb loss
-Fixes intake temp sensor location.
-Lets underhood heat out.
8. Belt drive with oil injection delete, D&R kit, 8T drivers: $1,050
- 10lb loss
- More responsive.
- Better attack angle to the snow and lets you run the Ice Age Elevate Kit.
- Pre-mix gas seems to run cleaner. I mix mine 32:1 even though 40:1 is a common recommendation.
9. Ice Age Elevate kit: $1500 (snowcheck price)
Anytime mods-Strip/streamline some "unnecessary" parts:
- Silicone any gaps in the bodywork/intake.
- Underhood foam and clutch side duct
- Skid idler wheels
- Trim Skag bolts
- Snow Flap
- SLP LED tail light: $20
- Loose 6lb