It seems like pretty dodgy ground really, as these units are not designed as propulsion devices and are not designed or certified as such. Here is an extract from the FAA TSO qualification standards:
The standards of TSO C77b apply to gas turbine engines intended to provide auxiliary electrical, pneumatic, or mechanical power to support airplane systems operations. These standards do not apply to gas turbine engines intended for aircraft propulsion nor do they address the integration of the APU
into the design of the airplane.