we have seen 16-80hp on mild engine builds, and a lot more on big builds vs. extreme ported heads. I don't dare post dyno sheets, they will be called b.s.
If you are wondering if you need one, you probably don't. If the head is the thing you back from more power they will help a great deal.
If poor tuning, undersized turbos, maxed out turbos, incorrect injectors, incorrect timing, incorrect cam, or other poor combo choices are there, you will most likely not see the benefit you are after. If your problem is getting more air in the cylinder, then you will see a great deal of improvement. We have seen over 200hp on bigger builds. As far as machining, there is some shavings in the heads from manufacturing. These can be removed by compressed air after washing them. I will be sure to include that in the instructions. One of the mentioned "bent valves" was from installing the exhaust valve 1.750" on the intake and the intake 1.850" on the exhaust which put the valve into the cylinder wall. Also we have had two people gall valves, I do not know if that was an assembly issue or an issue with the chrome stem. I hope to only sell assembled heads in the future to fix assembly errors.
I will be sure to include that as well in the revised instructions. I apologize that I did not previously cover possible shavings, making sure the exhaust valves are in the exhaust port, and that the valves need to be pre-lubed before installation.
Kawi600, be sure to use a .058" headgasket.