They Work!
We haven't made them in a while but the one I design would average between 6-10 hp at the wheels. And even more on some applications. EGT's are down too.
I should have patented these things. When I first came out with them a big diesel company was telling people they don't gain anything. Now they build and sell them. LOL.
By gutting the stock grid we didn't see a gain over stock because it creates a lot of turbulence, and the transitions to the head aren't smooth. Not to mention you ruin the stock grid, which you may need in cold weather.
Bang for the buck they are a great deal. Don't expect to feel 10 hp in a 7000lb truck though. However the increase will show up on a dyno or at the track.