The larger turbine (like a PW100 1.8MW in the dash-8 that this demonstrator is replacing) is at peak efficiency (~0.30kg/kWh output SFC) near full load, then it cuts back to 50% power while cruising where it also drops into a less-efficient SFC rate (~0.36) then down again to 20% (~0.45) for descent vs the "always at 100%" 1MW version which stays pretty much pinned at 0.30kg/kWh sfc sweet spot during all of the flight except descent where it also drops back and takes an efficiency hit.

Yes but then you're erasing some of your fuel efficiency gains on take-off by running a significantly less efficient APU to charge it back up. Also I think the APU is a built in unit designed entirely for in-flight loads (like an alternator on a car) and not intended to provide the kind of energy you'd need to even moderately recharge this pack after takeoff/ascent.