Xeon X5675 is a CPU with 6 physical cores, so your BL460c G7 has 12 cores. The fact that you're talking about 24 threads indicates you probably have Hyper-Threading enabled.
Intel says MKL is not a good candidate for Hyper-Threading, and that you may want to disable HT to get better performance:
http://software.intel.com/en-us/node/438654
Since you have two physical sockets, if your processes migrate between the physical sockets, the system will need to move a lot of data back & forth between the sockets to keep the L2/L3 caches in sync with each other. With a single multi-core processor, as in your laptop, the single L3 cache is shared between all cores so this problem is much reduced.
Intel says you should set task affinity masks to minimize this issue:
http://software.intel.com/en-us/node/438656
Fedora 18 also is likely to have a much newer version of GCC than RHEL 6, so it will probably do a much better job of optimizing the compiled code for the newer CPUs. You might consider installing a newer version of GCC, or perhaps the Intel compiler. At the very least, it will eliminate one performance-affecting variable between RHEL 6 and Fedora 18; but I would guess that the improved optimization would probably also provide a meaningful performance gain.