If you manage to get your 480 to do 821 on the core it looks like you'd be about 4% slower on average than a stock 580 @ 1920x.
People with 480s that do 850 on the core are probably close to dead even with a stock 580 or possibly a little a head. Of course the 580 will OC too, and people with those cards that OC decently will easily pull away from an OCed 480.
IMO an OCed 480 is still a viable card and I wouldn't buy one over a 580. I might hold onto one I already had that OCs well though.