KE=(weight(350g)/450240)*(velocity(280fps)*(velocity(280 fps)
350/450240=.00077736318 * (280*280)=78400
.00077736318*78400 =60.945273312 Call it 60.95 ftlbs

(300g/450240)* (315fps*315fps)
(.0006663113)* (99225) = 66.1147387425
Call it 66.11 ftlbs

Your calculations are correct. You generate more energy with with speed pro arrows. It is not uncommon for a heavier arrow to generate more down range energy. This appears to be a case contrary to popular belief. Did you actually chrono the diffrent arrows?