Agree with Ktmdad plus
We found the best lowend comes from the stock pipe. Raise the rear sprocket to a 48 or a 49. The 49 hinders WOT straights but allows the bike to be almost clutchless in single track trail. The V Force delta II reed block set on the low settings will give alot of grunt with a slight loss of overrev.
Last years setup had a DOMA pipe and Silencer (thanks ktmdad), V force (set on high), 48 rear sprocket for open courses, 49 for tight. Sharp jetting : stk main/#3/46 pilot and mr2 race fuel, timing retarded a big tad and a thin 01 cometic base gasket. This combo had about the same lowend as the stocker but now had the sweetest mid range. Junior is a big boy 150/ 5'6" and the bike would just sing.
Note: Eric Gorr had done some low-mid porting on our 99/00 that was real nice. that made the bike alittle short on top but so easy to ride.
As for the tank....I got 1.65 gallons in ours (24-26 miles on fast tracks)...How ???
Boiling water and air pressure. Drain and rinse tank. Bolt tank on frame (a must), put a screw in the vent in cap. I put the hot water in leaving about 1 inch space, let stand for about an hour. Now be Careful. I put a air line (low pressure) to the petcock. Cracking the petcock open a hair and allow air into tank. It will expand. Close the petcock and trap in the air. allow the tank to cool naturally. Repeat the process 2 more times. I did it every 2 days when I was waiting on some parts. We had it at 1 3/4 gallons but it shrunk a hair.
Best offroad suspension goes to MX Tech. Stock setup works well but once rider gets aggressive and faster...I have seen rear shocks crack/rear springs break and the front rims get hammered because of the weak front. We trashes a front rim and lost rear spokes. Had John Curea to the suspension, Junior got faster and the stock hubs and rims are fine. I am sure that others can do a good job if needed. In our series 6 out of 10 bikes are RM's. They do work well offroad. We run Harescrambles, Junior enduros and GNCC.s.
That is my take after 2.4 years on the RM 80/85 now the last year will be on a RM85L with the EG100 kit....now you want to talk about great power. As you can tell I am still impressed with this motor and snow is on the ground and Junior is driving me nuts to take him south......