HDF5 cannot do better than MPI I/O on your system. Usually HDF5 parallel applications have little overhead over MPI I/O applications on the same system. If MPI I/O performs well, then you should expect good performance from Parallel HDF5.
If you want to compare the performance of MPI I/O and Parallel HDF5 on your system, you can use the h5perf program that is built along with the parallel library. This is under the
./<HDF5 source code>/perform/ directory of the source code.