I have been doing some testing with arrow weights on my 04 Black Knight.
While it is only a 60lb model and I have a 28" draw.

I have found that using a light arrow 314gr will give me blistering speed 312fps it only gives me 68lb KE.

On the other hand a 400gr arrow drops the speed down to 280fps but yeilds 70 lbs of KE.

I did try heavier arrows but after 400gr's it got to the point of loosing KE

The bow is more efficant with a 400 gr arrow.

So I will be using 400gr this year