I asked this question on Archery Talk, its now 6 or 7 pages of people arguing.

Recently just changed my setup, was shooting a 417gr arrow at 248fps, now I am shooting a lighter arrow 351gr at 266fps.

I lost 1 ft pound of KE but gained 18fps and still not done tuning my bow. Will probably gain the KE back and maybe 1 or 2 feet.

Find a good happy medium between the two. I could go lighter but would drop my KE too low.