Actually, workloads that rely heavily on interprocess communication, or use a large amount of working memory will benefit from the single 12 core configuration from an architecture point of view.
The dual processor Nehalem/Westmere mac pros are actually NUMA systems, however OSX is not NUMA aware so the system is run with NUMA disabled and the memory striped across each node.
OS X may not be NUMA aware but dual CPU package systems means at least 4 more DIMMs slots. For appliactions that use a large amount of working memory having a large amount of memory is more important than nuanced NUMA performance tweaks.
But yes for memory sizes that can be easily match by both the single and dual CPU package systems there are some minor advantages.
This means that half of your memory accesses will take a latency hit as they have to cross the QPI bus and hit the other processors memory controller.
The problem is that is somewhat negated in the E5 systems in that the two QPI links are used to hook the CPUs together. There is no QPI link to the IOHub chip anymore. There is twice as much bandwidth as before. So while yes you have to cross QPI... but there are twice as many QPI links engaged in delivery.
The 12 core chip also NUMA aspects internally.
[from anantech article on the 12 core
http://www.anandtech.com/show/7285/intel-xeon-e5-2600-v2-12-core-ivy-bridge-ep ]
There are
three internal loops there. Frankly, OS X slacker approach to dealing with NUMA will have small impact there too because not going to be uniformly even point-to-point accesses there either (e.g., some L3 cache accesses are not going to be same). Single CPU package doesn't mean you don't have to deal with NUMA issues. If Apple wants to keep their head buried in the sand long term about NUMA issues they have to stick solely with mainstream Intel CPUs. Otherwise they can kick the can down the road, but it is coming eventually.