There is now a publicly available visual simulator for the selfish mining attack we outlined. To see the attack in action, set the "Node 0 hashing power each" to something like 40%, then click "Go" and then click "faster" multiple times like you have adult ADD, then check the percentage reported on the right. If it reports anything above the percentage that you selected, the attack is working. We get around 60% of the revenue when we let it run for a little while, which shows that the attack is real and successful.
The code was developed by ebfull, and funded by the Bitcoin community. I can't vouch for the overall correctness of the implementation -- javascript is beyond me -- but the results for high alpha seem in line with our simulations. Note that gamma is not controllable in this simulation, but is, I think, an emergent property of the underlying topology, propagation speeds and propagation tactics baked into the simulation (or maybe it's 0; I cannot tell). So, pick a high enough alpha for whatever gamma might be.