Truth be told, I don't know what Mo's shape is. I'd stake my life on it being no worse than 12:50, but it's a huge jump from that to 12:40, which I honestly don't know about.
I don't think dispatching sub 12:50 guys guarantees you're well south of there. If every race goes 12+ minutes at the bell, you're still fresh enough to use the vast majority of your closing speed, of which Mo, combined with his tactics, clearly has far more of than his competitors. If you're 15-20s off your ability on that day, there is plenty left for a scorcher final lap, the the amount of extra gain from being 25s off of day fitness (12:45 guy) vs 15s off day fitness (12:55 guy) is not that big of a difference when it comes to how well the last lap goes.
The reason why I'd bet a good sum on Mo being better than 12:50 is two factors. Number one is running 59:xx and 3:28 high in the same year. Pretty good combination that is likely to result in good 5k ability. Much more than this though is his consistency. If Mo were a 12:50 or god forbid a 12:55 guy...well bad days happen. If a 12:55 guy is having a bad day, he can easily show up on the line with day ability of 13:10. A 13:10 guy is totally maxed out, or dropped, by the time you even reach the last lap and would not be in contention. The fact that Farah always is there and always has a kick suggests to me he is in 12:50 or better shape.
My personal 50/50 mark would be on Mo being capable of 12:45.