I know with a skilled rider an 85 or 86 R will run with a 450 trx in desert riding conditions. To keep up with a good rider on a trx you would need what dammit suggested. pipe, silencer reeds bigger carb and porting. I'm going with the esr 310cc kit in one of mine soon, My friend Jessie has an 05 trx 450R he rides ok but I can take him out in the desert riding. he kills me in long drag races though. Be sure to get the suspension set up to handle the extra power
own:
1986 250R- ported polished, carb mods, Esr atc-5 pipe and silencer
1986 250R- mild porting, carb mods (Clean!)
1985 350X- all stock (super clean!!)
1985 350x- desert project in the works
1986 250sx- Mostly stock
1985 200x- (pretty clean mostly stock)
1985 Yamaha tri zinger 60