To make a HD5870 get a good 10-15% performance gain, doncha need LN2 cooling anyways? The only real question is will it actually show up and run, seriously, the FX5800 and HD2900XT had more dignified timeframes.
You need just aircooling plus softwarte voltage increase(maybe) for a 10% increase,and many cards will do even +15% (thats still just 977Mhz core, some are doing 1ghz+).
However ,taking into consideration 10.3a catalysts even that much wont be necessary..
From legitreviews:

I certainly hope nvidia has some driver tweaks waiting to be unleashed also.