It is intriguing, and we have one D700 Mac Pro for test purposes. At the time we ordered the Pros for the prototype rack that is the subject of this article, we found that other parts of our pipeline were preventing us from taking full advantage of the increased GPU performance. So we ratcheted down to the D500.
Keep in mind that either of them offer significantly higher gflop/s per system than the best GPU ever shipped on a Mac Mini (480 vs 2200 vs 3500).
However, we have fixed bottlenecks in our pipeline as we identified them, so it is probably time to re-evaluate. I actually just had a conversation with an engineer a minute ago who is going to jump on this in the next few days. Higher throughput and better $/gflop is always the goal, just have to make sure we can actually see the improvement in practice.