There's more to it than just brace height & power stroke. I haven't talked with Steve about this, but I'm pretty sure that a 68" 50#@28" NM Classic doesn't have the exact same limbs (stack thickness, etc.) as a 68" 50#@28" Whisper (reverse handle). Assuming the same limb profile, width, and taper rate, the limbs on the Whisper would have to be a tad thicker (deeper) to give the same draw weight at the same draw length as the Classic.
If you could take a 50#@28" Classic, and simply move the riser to the back side to make it a reverse-handle bow, you wouldn't have a 50#@28" reverse-handle bow. The draw weight at 28" would be a few pounds lower.