0.5 HP maybe? The only thing it might really change that could effect HP is the rocker geometry, and how much it will change (if at all), I don't know.
It would be nice to have for the ability to get the hydraulic lifter preload set where it should be.
BTW, did you toss that bad lifter? If not, how's about sending to me? If I get time, I would like to try to diagnose the issue.