HP gain with multi angle valve/seat

Average amount of what? Power added I guess? Man, that's a TOUGH one to answer since there are so many variables involved. BUT........

A lot of people don't realize that every single valve job in our older cars came right from the factory with three angles on the seat. So when a machine shop charges more for a "three angle" valve job, you're getting a stock valve job. The only thing different they do is adding a back cut to the valves.

Now, modern machines like the Serdi and Rottler like my friend Richie has that he did my slant 6 head with are a different story. They have cotters that can cut several angles at once. I think mine ended up with eight angles in the head. Crazy technology.

I think in terms of what it's "adding" would better be looked at in CFM rather than horse power. But maybe that's what you meant?