I honestly thought you'd see a little bit of a change in trajectory - but nothing crazy.Maybe just a nudge. I'm guessing the speed loss is under 20 fps.
Just seems like you should be generating more than 76# KE at 78# draw weight. Either way, definitely plenty of energy for any North American game.
As the arrow weight increases, added weight has a diminishing effect on speed - and trajectory. For example, if I stacked 65 grains on my 255 grain arrow, I'd see a big speed (trajectory) change. But, if I was sighted in with a 500 grain arrow, and bumped up to 565 - the difference would be much smaller.
http://www.huntersfriend.com/2007-Carbon-Arrows/arrow-selection-guide5.htm This chart kinda shows how that phenomenon plays out.