Just to piggy back on 6000rpm....

The valve springs is more likely what you had in mind. They get more or less mushy at the higher rpm levels. Meaning that they wont fully open and close with each cam revolution as intended, commonly called valve float. Filling the cyliders is not an issue cause we are cramming it in there with the blower. Another issuse that may or may not be on this motor is rod bolt stretch. With high rpm comes huge forces to be dealt with. Not sure if the factory stuff would be able to keep it all in check.