I can't explain why they would bump their recommended PSU requirements. I just know that there isn't a chance in hell you need a 750W PSU for just a 280. Now if you were to crossfire 280's then yeah you would be needing that if not a bit more for extra headroom. Even if you're overclocking your CPU and that 280 I still can't see it tripping the OCP on a 500W PSU unless its a cheapo unit.
For comparison's sake, I am running an overclocked 4930K, overclocked 290X and a overclocked 295X2 with a custom water cooling loop and my power consumption at load is a shade over 1000W but I never have seen it even hit 1100W. (Firestrike, Firestrike Extreme, Firestrike Ultra, gaming in general or stress testing with AIDA64 Extreme). Personally I wouldn't worry with using a single 280 on a 500W PSU.
So with an overclocked CPU and three power hungry overclocked GPUs, mobo, pump, case lighting and misc other bullshit I still get reasonable power consumption. I measure at the wall outlet using a Kill-A-Watt meter. Them shits are cheap and convenient.