Look
here.
Buy
here.
Karl Otto Noelle is the brains behind Alpina's famed turbo technologies that are the basis for every BMW turbo car today (135, 335, 535, M5, etc.) Doesn't get any more credible than that. I have Step 3 on my car and it performs beautifully and has zero issues.
Ron Trochelman is an absolute perfectionist, enthusiast and highly connected BMW performance expert. A perfect partnership with Noelle. Runs the same products on his personal F10 M5. Highly credible as well.