I've had several opportunities to run my brother's original Effin-A (which started as mine, sadly) and I've always liked it. The original review is here (principles are the same, different caliber), https://kitup.military.com/2011/11/effin-compensator-evaluation-adjustable.html.Predator Intelligence recently sent me the AK version to T&E, but I haven't had the chance yet. I don't own a Kalashnikov yet, and the two guys who usually help me with AK reviews are both out of pocket (one is deployed, the other unfortunately is the primary on a pretty bad drug/child abuse case).
So, since I haven't had the chance to do a proper review on my own, I thought I'd bring this to your attention in the meantime.
The Effin-A is available here: http://www.predatorarmament.com/effin-a-compensator-by-ares-armor/.