For anyone with any doubts about these statements read Dirt Rider the last two issues have had in depth articles on valvetrains and everything they said can be found elsewhere.The new 250 4st motors the replacement interval is cut in half 20 hrs.They run at 13,500 rpm vs the 11,500 rpm of most of the 450's.Valvetrains are the same so 2000 rpm extra doubles the wear.Now go the other direction and 10,000 rpm should just about get you double the time.Not worth the power loss for me though.
I have doubts, major ones at that. Yamaha and Honda make some of the most powerful sport street bikes around. These motors rev as high if not higher than any atv engine. Figuring an average of 60mph on a street bike when you hit 3,000 miles that would be 50hrs of engine time. Those engines go MUCH longer than that before the valves need to be touched.