from a prior post:
I don't have a '98, but I do have Eric's 265cc setup on my '01 CR250, with low-mid porting. Horsepower/torque were 45/32 at the rear wheel, with the hp. peak at 6500 rpm. Very nice smooth curve, with almost no hit at all...just smooth progressive power.
Basic setup at dyno time:
EG 265 cylinder w/ low-mid porting
Stock pipe/silencer
Stock carb, 370 main, stock needle in 3rd clip, 25 pilot jet
VP C12 mixed 40:1 with Maxima Castor 927
Dyno runs were done this summer, around 80F/80% humidity or thereabouts.
Just a data point...hope it's helpful
http://www.dirtrider.net/forums3/showthread.php?t=68152&highlight=dyno+265