The cams come in 2.5 inch draw length increments. I have heard, but have not personally tested the theory) that if you get a module that is at the max for your drawlength (26 - 28.5 in your case) it will be the most efficient and the fastest.

I shoot a Razortec at 28.5, 72#s, 65% mods, 360 grain arrow (3D arrows) and get 284 fps. I was getting 281 fps with the 75% mods.