What you pay for is R&D. AA has plenty of experience tuning our cars, and the proof is in their staff and products.
Origionally, I was skeptical of AA, as I am with any small company. Working with their staff has resulted in my total endorsement. Dinan, AA, Bavauto, Turnermotorsport...they all offer tuner products and are quite reputable.
As for Active Autowerke, I'm convinced I made the right decision in working with them to develop software for my car. Unlike Turner, they were willing to take my laundry list of mods and a several dyno sheets, and build software. By next weekend, I'll report the results.
You won't have a new car with one of AA's performance kits, but you will have a boost in performance that will be very noticeable, with the reliablility of a trusted tuner.