It's actually 1 to 2 hp over 4k. They are a very bad idea, I don't care who's brand they are, equal length shorty headers suck.
MAC are fine. I have installed them for 15 years, they fit and work fine. I've taken them off and put them back on after years of use with no issues. Most people just don't have the knowledge of how to get things on and off without issues, so they post all over the internet blasting a brand because they have issues with it, when in reality, they just lack the experience to handle them. The easier to work with products usually get better ratings online, doesn't mean they are better, just easier for a beginner to work with sometimes.
Not bashing anyone in this thread, just saying I don't see much difference in headers from an experienced installer point of view. They all have their drawbacks and bonuses. I ran MAC ceramic 1 3/4" shorty headers on my race car for years, and they still look nice and I've had them on and off a dozen times with no issues.
Bottom line, buy what you think looks nice and can afford. Remember, cheaper is not usually better.