I can't say how much HP you get from it, but I can definitely tell you it kicks a**. I just installed one this weekend and I love it. Before the VAK, my torquless wonder duratec would get a chirp, maybe two when stomping it from a dead stop. I did a test run yesterday in 80* weather(Florida winters are brutal, aren't they?) and got a nice 1-2 second solid tire screech. The VAK is definitely worth it, and I would recommend to anybody. Also, it's something you can install yourself, unlike the UDP I got from ShoShop.
Also, on gen 3's, it cuts out the alternator, power steering, and a/c(I think).