By the way, since you are so concerned about power loss and voltage drop - I hope you have already seen the datasheet of L298 in detail.

There would be voltage drop across the high-side and low-slide transistors in any H-bridge.
If, for example, your motor current is 1A, typ, the datasheet of L298 says,
total drop across these transistors could be 2.5V approx (typ) and 3.2V(max) at 1A. (please see datasheet for details of these values)
I hope you have already tried your motor with intended battery and the L298, and verified that you get required torque/speed in final setup.