The 6.5 PRC (270, Creedmoor, et. al) rely on one powder to approximate larger cases... the problem comes if that powder will not deliver accuracy in the smaller case that one of many other powders might provide the larger case.
At the end of the day, the bigger engine wins.
My philosophy is to accept a smaller cartridge for what it is and not try to make it something it isn't. A 308 Win will never be a 300 WSM, wonderful (and in many ways better) cartridge the 308 Win may be.
