There is a curve where you can absorb near maximum energy while still maintaining decent speed. You'd have to plot that out on a chart shooting different arrows through a chrono, or you could use a program like OnTarget2, and it will do it for you to some extent. For me, I'm shooting a 445gr arrow out of a 71# bow. That arrow is zipping out there around 286fps, not super fast, but it hits REALLY HARD in comparison to what a 350 arrow would.....

And I agree with most everything Bigbulls said, especially the energy absorbed by the arrow vs. the energy absorbed by the bow. I believe if you took an HONEST poll of guys who'd had a limb failure, you'd find that at least 75% of them were shooting LIGHTER arrows, less than 6gr/#, and some less than 5gr/#. Speed kills alright, but unfortunately, sometimes all it kills is your bow!!!!