Renault : thousands of employees, including Renault Sport top engineers. Thousands of hours of work and development, CAD softwares that cost millions and decades of expertise in racing and developping engines and manifolds, among other stuff. Currently producing the best price/performance ratio cars on the market, they are a reference.
K-tec : small company with limited means, certainly talented people, but far less experience and knowledge, definitely less working time to develop the parts, etc etc.
Inoxline performance : one guy with years of experience, using the OEM manifold as reference for the shape, size, path of each tube, but simply making it out of the best stainless steel available on the market, and removing the only annoying thing on the OEM system : the cat.
So i respect K-tec and their work, but as they say : "why trying to reinvent the wheel"? The OEM manifold has a very very good design, only flaw is the material chosen to keep the production price down, and that welded cat. So why bother doing it different??