On my 5870 it jumped 16 mh from 385 to 401. I'm using the poclbm "old" kernel + the other +3% patch (that gives me only 1% of speedup :-( )
Weird, what's the rest of your setup? I've listed mine above and this mod decreases my performance on every 5870 I have tried, mix of ubuntu/windows, ati versions & sdk.