Chill out y'all. no sence in getting in an arguement, just a misunderstanding.
As far as RPM. Common rail engines seem to plateau at about 5200 RPM. Much above this seems to be giving us fits with cylinder pressures getting out of balance. I have worked with an independent injector component manufacturer and their research facility to come to this conclusion. That doesn't mean that is the absolute limit, but where we were comfortable going to. There are advances being made every day in injector technology, and i am sure things will improve.
As far as a mechanically injected engine goes, you can go as fast as you can get fuel to burn. Honesty, I see no need for anything above 6800-7200 RPM, but I believe the hard parts living is going to be a greater concern. We'll just have to see how things go this winter.