The old one had two CPUs so 130W went to one of them. You can't put two of these FirePros in a 2012 Mac Pro. The Dell Precision has a 1300W PSU.
Right but that's up to the developer. They should be using OpenCL.
The space saving is quite dramatic. I don't think they quite got that across in the presentation:
The old MP might not have been able to easily mix slots and Thunderbolt. The old MP could not run two FirePros internally. You can say similar things about the rMBP and the cMBP. You can technically do more with the cMBP like put your own RAM in and HDD and you have an optical drive. It doesn't mean much though if the unique things the old one did aren't that important.
I freely admit that it wouldn't have been faster and more expandable.
It could have been built faster but it wouldn't be if it had the same restrictions the design has had for the last 10 years. Also, when you say 'more expandable', you mean you could use 3 devices at a time instead of 36. I'd say 36 is more than 3. The slots are faster but I haven't seen any real world scenarios where it's a measured problem.