The internal shape of the hat has a strong effect on mixture distribution (at least, on a 4 bbl.). Some of the "looks cool, must work well" hats favor front or rear barrels, and not only make for weird plug readings and knock sensor reports, but less power. This is evidently not easy to predict, since the number of commercial hats that don't do well is fairly large.
Here's one that supposedly works, notice the larger radius above the air-horn, and the separator.
