Maybe this will shed some light on the subject...
A given engines' "redline" is what the engine was designed to run at for it's ''service life", service life being defined as when the engine is no longer within factory specs..
So, if the engine has a 6000 rpm redline and a 500 hour service life, then if run at 75% of redlline, it should run 25% longer before it needs to be rebuilt. 50% of redline all the time would result in a 50% longer service life ect.
HOWEVER since these frame mount engines can vary wildly in quality control, one can only assume that "factory specs" are just as wild of a variable.
Run 'em where they feel/sound good, and don't worry, be happy. If you get 1000 miles from one, you got your money's worth.