Wooly,
It's actually pretty easy to estimate the possible speed numbers for your set-up. Since a bow only has one efficiency level and KE changes little from one arrow weight to the next, coming up with estimates is easy in your case since you have already established a known chronographed reading.
KE will only increase slightly from the lightest to the heaviest arrows, and in your case where you want to decrease arrow weight, it will DROP slightly over the changes to a progressively lighter arrow.
As speed increases, arrow weight decreases and KE follows accordingly with only slight changes from weight to weight. KE from my testing usualy only rises 1-2ft lbs over the long haul from light IBO specs (5gr/lb) to a 450+gr arrow weight.

Your known numbers are below the factory levels due to the string loop, and possibly the Whisker Bisquit robbing some speed(and maybe a peep or string silencers?)In any event you've lost 9-10fps over BowTech's bare string level.

Here are the numbers as I see them......

FACTORY:
350gr = 281fps = 61ft lbs of KE

YOUR KNOW LEVEL:
500gr = 228fps = 58ft lbs of KE

ESTIMATES+/- probably 1-2fps)
475gr = 234fps = 58ft lbs of KE
450gr = 238fps = 57ft lbs of KE
425gr = 243fps = 57ft lbs of KE
400gr = 253fps = 57ft lbs of KE
375gr = 262fps = 57ft lbs of KE
350gr = 272fps = 57ft lbs of KE

There's a bunch of good light to med weight internal component shafts on the market now......I personally like the Carbon Express "CX" and their "Terminator Selects", but theres also Carbon Impact Fat Shafts,Gold Tips etc.......