Most bows won't be as efficient when the limbs are backed out this far. So if you took a bow that had 70lb max limbs and shot at 70lbs w/ the 350 grains it would most likely be faster than if you backed that 70lb max bow down to 60lbs and then shot 300 grains out of it. There are very few bows on the market with limbs that load enough to produce a similar amount of energy when 10lbs is taken out.

Now take that 70lb max bow at 350 grains, and a 60lb max bow at 300 grains and most modern bows will be pretty darn close provided the same draw length, cam tune, and similar accessories on the string.