SQL Server 2012 x64 - cannot safely allocate more than 50% RAM
I'd check the perfmon metrics for per-cpu CPU busy, as well as the perfmon metrics instance-wide and per NUMA node for PLE, database pages, etc.
Two recent SQL Server KBs


FIX: SQL Server performance issues in NUMA environments
FIX: You may experience performance issues in SQL Server 2012


However, for some workloads, especially with higher core count, high concurrent query count, and lots of database disk IO, using trace flags 8015 (disable NUMA support at SQL Server level) and 8048 (remove per-socket query memory allocation botlleneck) will provide even better results than the fixes contained in SQL Server 2012 SP1 CU4. (I confirmed this on our test equipment with simulations of our workload - YMMV)

Trace flag 8015 deserves thorough evaluation before deployment.  Memory affinity and attending lower memory latency is sacrificed for managing a single large bpool.  It also results in one lazy writer instead of one per NUMA node, and the idea of affinitizing a connection endpoint within a single SQL Server instance to each NUMA node also disappears.  But for some workloads the benefits are undeniable.  Don't use 8015 without 8048.

I've never seen or heard of any measurable cost to adding trace flag 8048, and if CMEMTHREAD waits and associated spinlocks are triggered by query memory allocation - its the only reliable way to eliminate them.