I'm not an engineer or mathmatician, but I do know/can hear when my engine "bogs" down, and I can say for sure that heavier blades require a lot more resistance from the grass to slow them down than lighter blades do, and the lighter ones will recover more quickly. The difference in slow down, and thus recovery time never seems to equal the amount of extra drag required to slow them down over the lighter sets in the first place. Another point that needs to be made is that once the baldes are spinning full RPM, it doesn't take any more HP (if even as much) to keep them spinning due to their extra mass/inertia. Also, a lighter blade wil slip on the belt quicker than a heavy blade will since it is more easily held back by thick grass all other things being equal.
All the theortetical/opinion stuff aside for the moment, the G6's for my machine are the same width, and thickness as the stock blades, and the lift isn't any more pronounced than the OEM high lifts has, so spinning them won't be a problem (for me anyways).