In addition to the benchmarks, I might also suggest to look at the statistics printed in the log at the end of a run (A very short run might be enough). That will indicate how much your are loosing in communication, which is probably quite a lot! As a general idea, in solide state MD, I found >10 atoms per core very, very small! I might be wrong but curious, so keep me/us informed on the results of your benchmarks! 
Also, another thought. Load balancing is based on the number of particles only. In some specific cases, could it make sense to also consider the local structure (directional flow, localized complex interface…) in order to distribute the cores?