To my best recollection (24 yrs ago in SF340 flt test) the AC wild freq generator is on the Prop Gear Box (Gearcase), not the core AGB which has a DC starter-generator.
Thus the indicated torque (measured at the PGB input axis) must drive both the prop AND the AC generator; In other words the power at the prop is reduced by the amount of the generator power when it is loaded, assuming equal indicated torque. Whether this is a significant figure I'll leave to someone with more current knowledge of the system.