Originally Posted by xluben
For the 20G XTR I think that is mostly unnecessary. It's already running out of breath up top. Maybe some head porting would help, but raising the rev limiter on it's own won't really get you much, especially on meth (vs E85). I don't know that valves/springs would be worth it. I'd discuss this with a more experienced builder though.
There are benefits to headwork outside of power such as throttle response and overall smoothness of acceleration among other variables.