The posts on this have been a fun read! I will add a couple comments in the spirit of the intent of the OP question.
What OP question boils down to is how to get the best performance from the snowmobile. The answer to that is affected by quite a few variables. Number one being the terrain and the snow conditions. Going rip-zinging down a drag strip or across a wind swept lake is different than speed cruising across the fields and rolling hills which is again different from speeding around winding groomed trails and finally the steep and deep.
All that said, there are a couple things to do to ensure best possible performance out of the machine you have. Before you start, know the snow conditions that you are tuning for because the setup will differ.
- use good oil and fresh fuel
- ensure the carbs are jetted and tuned, or EFI is mapped, for best possible fuel ratio and performance.
- CLUTCHING. This is number one and by far where the most time is to be spent. The clutches must be clean, components in good condition, and operate smoothly with no binding. Adjust clutch weights and springs to achieve a steady peak full throttle RPM that is just shy of the peak HP rated RPM of the specific engine in target the snow conditions. Lots of test rides and iterative changes are involved
- final drive gearing. Again, depends on the target snow/terrain conditions. Sprockets can be changed in the chain case as well as the track drivers size. When gearing is changed, clutching adjustments have to be redone.
- Track alignment and tension. Misaligned or overtightened track robs HP and speed. Hifax needs to be in good condition as well as the bearings in all of the track rollers. Large diameter rear rollers really help to reduce track rolling resistance and drag.
- Suspension. Adjust track limiter strap and shock/spring tension to get the best weight balance pressure between the track and the skis. when adjusting balance point, the rider must be on the machine.
- Ski alignment. Ever so slight toe-in / toe-out, whichever your preference. For winding technical riding I like a smidge of toe-in, for high speed rip-roaring I like toe-out to reduce darting. Sharp low drag skags are important. Also ensure the proper size skis (width/length) as they can be a huge drag, or submarine, of the skis do not provide enough support for the weight and balance of the machine.
Get all of the above right and the sled will be doing everything it possibly can and should do, in terms of available performance. Further from that will not be anything that has direct effect on making the existing sled as-is any faster. The next stage of mods/adjustments would all be focused on adding power to the engine. The only way to add power with absolute certainty of results and reliability is to add displacement. That means install a bigger engine. Other than super charging (turbo), much of the add-on offerings provide marginal gains at high cost per HP and sharp drops in reliability. There are no better gains than replacement with more displacement.
Hope that helps! ... In some way.
PS: If you want to go fast in a straight line. .... Get yourself onto either of the Storm or MachZ triples. Wear grippy gloves, a good helmet with wind visor, and ensure you have gone to the toilet before the ride.