i think the limiting factor here is the size of the charger not the engine.

Obviously the nature of the engine with regards to squareness plays a part, as well as rod to stroke ratio, but with the right top end they can be revved safely past the stock settings. it is only heresay but I have heard tell that these motors, in their design phase, where tested to 7k rpm worth of stress. The setting to just under 6k is both a saftey margin and economical factor.

ford are reknowned for detuning their motors someting fierce to accomidate relliability among the lowest common denominator.