I have run the same calculation on EVE and JUQUEEN. On EVE the optimal number of cores, larger than which the speedup became flatten or decrease, is around 300. On JUQEEN the number is 500 cores or more. Of coarse, we can say it's due to the hardware difference as that JUQEEN is super computer and has something like shared memory architecture, while EVE has the distributed memory with infiniband connection.  However, only XLC works for mpi job on JUQUEEN so far.