I've been looking over the Intel data sheets for these processors and it looks like the E5-2667 v2 is indeed the highest performing processor in the lineup. However, that comes with a major downside. It has the very worst thermal properties. It requires more airflow than the other chips. So the fans should be kept at a higher speed with that processor installed. That's not an ideal scenario for those who value silent operation. And due to the thermal constraints of this Mac I think it's best avoided. It looks like the 10 core E5-2690 v2 would be fine. It appears to have slightly better thermal properties than the stock 12 core. Intel lists the same thermal properties for the E5-1650 v2 and E5-1660 v2 so the latter may be a good upgrade for those who want the best single thread performance. For 8 core I think it's best to stick with the E5-1680 v2.