Yes it is possible that CPU A will need to access memory connected to CPU B (remote memory access). The performance delta between local memory access and remote memory access is benchmarked at Intel for both latency and bandwidth and since usage models vary, (some) BIOS versions have options to modify the resource allocations for QPI. If you are configured for "non-numa" where 50% of all accesses will be to remote memory then you may benefit in changing the QPI resources for more remote credits.
Have you tried PTU? If may have what you need for basic statistics on the QPI link between the CPUs.

http://software.intel.com/en-us/articles/intel-performance-tuning-utility/

http://software.intel.com/en-us/articles/optimizing-applications-for-numa/

Ye,

The latency depends a lot on processor type, platform and memory. Therefore, it is difficult to provide general numbers. However, it is fairly simple to measure the latencies on your system:

LMbench (available here) contains a microbenchmark "lat_mem_rd"to measure the memory latency. With the tool "numactl" (part of libnuma), you can use it to measure the latency of the local and remote memory on your system:

numactl --cpunodebind=0 --membind=0 ./lat_mem_rd -t 1024

numactl --cpunodebind=0 --membind=1 ./lat_mem_rd -t 1024

Kind regards
Thomas