What kind of horsepower gain do they have over the competition?

 

I don't think they produce a significantly different horsepower figure, but rather what sets it apart from the competitors (Pace Setter, Jet-Hot, Hooker, etc.) is the build quality and materials. Kook's headers are made from stainless steel, unlike the other brands that use mild steel. What does this equate to? No rust. In addition to being rust-proof, they come ceramic-coated as well. Basically, you're paying extra money for a product that will last. I'd rather buy one set of Kook's than 2 or more sets of another brand of long tubes over the years...

 

plug swap in 45 min, without having to remove anything from the motor...all from the top too.

 

Actually, I don't think Kooks are coated, although you can certainly get them coated. Coatings supposedly don't stick to the stainless Kooks as well as mild steel headers - but the stainless headers' heat properties and rust resistance make coatings unneccesary. I've heard many times from many people that HP is similar with Kooks and most other LTs. The reason for buying Kooks is the quality of the materials and workmanship that go into their product. As many people on the board have said, when you buy Kooks, you're buying your last set of headers.

 

If quality is one of the dominant factors in your decision, then yes, they are worth it.

 

The cera-metallic coatings improve performance, and keep some of the heat out of the engine compartment. They are not just for "rust". Stainless resists oxidation, and maintains its strength at higher temperatures, but it doesn't keep heat inside or limit heat flow to the outside, compared to mild steel.

I'd be surprised if stainless steels presented a problem to adhesion of the cera-metallic coatings, since they were developed for use in gas turbines, where the use of high alloy steels is common. Many people get their SLP S/S headers coated, and I haven't heard of any problems. Worth looking into some more.