575fps with AAF's is 11.6ftlbs. As long as that is the absolute limit of what the rifle will produce, I feel that is a decent enough safety net, legality wise, seeing as AAF's are the most efficient pellets that I've found to be available. However, that's my choice, and if you would be more comfortable with a lower velocity/bigger gap to the limit then it's your call. But to my mind, the whole 'power to limit' situation is a bit of a gripe anyway.
Yes, the lower the power, the more shots per charge (SPC) you'll get.
I always aliken it to driving a car with a full tank of fuel........if you drive it flat out, you'll get less miles per tankful and empty the tank sooner, than if you drove it at a lesser pace.