HUM.......??????
A&L Lethal Weapon is the best tranny ever made it will hold up to anything..... Not Really.
Ok -
Traction is a point to think about along with all the other points mentioned.
A properly adjusted power clutch should take up most of the 0 to full throttle starts. The roughest thing on the belt and gears is full throttle landings off of jumps where the tires end up spinning two to three times faster then the forward motion of the car. So that when landing they need to slow down by 50 to 60% at once and the motor tries to keep them going.
I do think that a 9T brushless would be too much, but at the last Worlds Chris went to way back when, I believe he ran 9T and 10T quads on 2000's with out any belt problems.
There is a belt adjustment we used in the LW's, you take scotch type and wrap it around the middle belt idler pulley in the tranny. You want to do complete revolutions - start and stop each application at the same point on the pulley and trim off any type even with the outside of the pulley. I have personally put as many as 4 complete layers on the center pulley to tighten the belt tension. When you put in into the tranny you want to install it so that the belts running over the pulley is rolling the type onto the pulley and not trying to pull it off. This is the hard part to describe, anyway thsi is how we use to get 200 plus runs out of the belts in our team's trannies, they were told to also check the belt tension and keep it from getting too loss.
Hope this helps, depending on the driver a LW could take a 9T brushless - but I would not advertise it could because of all the kids out there that drive like me with just an on/off switch in place of a speed control.....
If there is a FAQ section or tech section, someone who understands what I just wrote and can make it more understandable to others is free to rewrite and stick this tune up tip for the LW in it.