If we assume 100 grains of virtual mass (thats pretty typical) then a bow that launches a 525 grain arrow at 220ft/sec (which is 56.4 ft/lbs KE) will launch a 350 grain arrow at precisely 259.3 ft/sec which is 52.3 ft/lbs of KE. Thats a 7.4% drop in energy launched. This difference will increase slightly at longer distances due to the fact that the heavier arrow because of slightly greater momentum will maintain slightly more of its energy.

If bows transferred 100% of their stored energy to the arrow then it wouldn't make any difference what weight arrow you used as far as energy launced goes but the reality is that they are not 100%. It depends on the bow of course but values of around 83/84% efficient are typical for launching 500 to 600 grain arrows and values around 75/76% are typical for launching 300 grain arrows. The lighter the arrow the less efficient and therfore the less KE.

A good way to explain the efficiency change is to think of throwing a ping pong ball verses a golf ball against a window. They are roughly the same size but because the golf ball is heavier you can transfer more energy from your arm to the golf ball. Don't try it but I bet you can break the window with the golf ball but I doubt you could with the ping pong ball.