The slp performance kit is really good bang for your buck. I put that on mine and it seems to run or out run most of the new iron that comes out. I have found a team secondary with the right setup provides some gains and having all the clutch updates is crucial as well. Just having the clutch updates were night and day. The first year I owned that thing it was a pretty sad excuse for an 800, actually it was a pretty sad excuse for a 600. If you get an x or xrs ditch the track. A powdermax or challenger extreme will outperform the challenger light in about 95% of conditions. But before upgrades make sure the clutches are right. That will make or break the 07s.
Clutching all the way. Talk to DJ or buy the 2007 race book and do some reading, and play with it yourself. The book was about 80 bucks if I remember correctly
Mods?
Move the ECM underneath the head light to protect from water corrosion.
Put dielectric grease in every connection.
Remove hoses under the fuel tank and re-route in alum tubes to the running boards.
Install cross brace on PTO side to stiffen structure.
Remove plastic clutch shroud.
Buy stock 163 track from 05 1000 and rails, 151 is too short.
DJ secondary helix.
Any primary clutch except the TRA. (Comet Cat Polaris Yamaha)
BMRP can.
Filter cover backside of air box to prevent Kevlar from going through engine.
Install 08 thermostat (cheap about $70 total)
Replace RAVE valves as the "ears" will break off.
Drill steering spindle at ski attachment and install grease nipple.
Gear up to 21/43.
Install a set of skis that suit your riding area if you must.
Install steel back bumper so you can pull other sleds out that are stuck, or broke down.
Leave everything else stock, everything. No vents, nothing.
You will then have a very reliable, fuel efficient, top performing stock sled for not much money, with a stock Rotax motor, that will last a long time. There is a lot of potential in the BRP sleds that BRP isn't willing to spend the time to give to the customer. You have to do it yourself.