As others have said, Packs OFF provides additional engine thrust, thus the RTOW may be increased.
Yes, it does save fuel, at any time that the fuel burned is providing for OTHER than engine thrust (e.g. Bleed Air, Generators, Hydraulics) the overall fuel use increases. Of course, turning any of the generators or hydraulic pumps off is unthinkable, but the example is there. At low altitude of course, pressurisation is not an issue, but perhaps a comfort issue in hot climates, which, coincidently, is when you'll most likely need to do a Packs Off Takeoff.
Reducing bleed air for pressurisation / air conditioning is a current trend for fuel saving, e.g. the now-common use of recirculation fans to reduce bleed air requirement.
Regards,
Old Smokey