I don't have it but I can tell you that you WILL get some gain from it.
It modifys the fuel/air signal. If it's tuned correctly, it can get you some HP. I don't know exactly how much.. maybe 5 to 10 hp.
You can set the S-AFC to consume different amounts of fuel & air at different stages or in intervals of 500 rpm.
I don't know much about it though, so.. anybody who has one please jump in now..