The 8kWh/kg number published by Phinergy is actually the theoretical limit of the aluminum itself (does not include the cathode, electrolyte, CO2 barrier, cell/pack enclosure, cell interconnects, real world inefficiencies). Wikipedia references a paper that says 1.3-2kWh/kg for current and projected Al-air batteries. That gives 50kWh for a 25kg pack. Sure that's much better than Li-ion batteries, but is that good enough for range extension?
Power is the bigger issue. Theoretical specific power is 0.2kW/kg, so that only provides 5kW of power for 25kg, definitely not near enough power to act as a "range extender". A small car would need 15-20kW for a generator for highway travel. I suspect given the "1000 miles" of range claim by Phinergy, the pack is closer to 200kWh and 100kg given 2kWh/kg, to provide the 15-20kW necessary to serve as a range extender. And a 100kg (220lb) pack is not practical for one person to lift (although possible with two people).
http://www.greencarcongress.com/2010/03/alair-20100323.html
http://en.wikipedia.org/wiki/Aluminium–air_battery
http://en.wikipedia.org/wiki/Genset_trailer