*( deep breath )*
Need to add a thermostat from 08 XP 800 ... eliminates bog in deep snow and climbing in good snow
Clutching, DJ has a good kit for the RT
Eliminate the rear cooler... 05's WAY over cooled, the rear cooler is easy to remove. Helps to keep the engine running in the correct temp range to prevent the bog mentioned above ( sled will drop into warm up mode if the coolant get's to cold ... like turning you choke on ) also will reduce the sleds weight by about 7 lbs including the coolant it contains. In 06 skidoo did this from the factory and reduced the capacity of the tunnel cooler to boot.
07 mid altitube maps is a must, regardless of altitude.
BR8ECS plugs ... nothing else
Verify it's idling around 1,500-1,750, if not set it to there.
Clean your exhaust valves, do it now and at least mid season to prevent them from getting gummed up. Most of the time you'll be able to reuse the gaskets but have an extra set in the garage as this is a maintance item and you'll need to keep up on it if you want the sled to perform.
Make sure your running a good skidoo 189 belt... NOTHING ELSE.
Pull and clean your throttle bodies, and the reeds. If the sled does not have pods installed they will be full of belt dust.
Put some sort of air filter system on the motor, if you search there are alot of guys that just remove the air box, vent the sled very well and install some pod air filters .... this is what I did and my sled ran great.
Remove the stock facet fuel pump from under the motor so you can change it on the trail, and carry a spare. Or replace it with the Chevy lift pump ... don't recall the part number. The stock pumps are junk and need to be replaced, if they go out the sled will run good with a full tank and then lose performance after around 3/4 tank. It's also recomended to install a relay to power this pump ( this is the low pressure pump, the sled also has a high pressure pump located in the tank and it is mosty never a problem.
Regear it, 21/49 should be stock with the 162, and 23/49 for the 151. I'd go 20/50 if possible or 19/49 but the bend in the chain is pretty steep.
Buy a new battery, every year. If the sled is electric start then you might get two seasons out of it but if it's just got the little battery then don't even mess with it. You can get a new one for around 30 dollars and it's worth it to if you were to have one bad trip because your battery was not holding a good charge ( sled off , hit RER/START button, you'll hear the pumps cycle, then check voltage, 12.5 or better your OK, anything less scrap that battery )
There how was that?
Maybe more people will read it and chime in to help you out with anything I missed or did not describe very well.