The performance should be comparable as auto parallelization is base on OpenMP implementation. We may be able to anylyze the issue if you provide sample test case and compiler options.
This assumes that the auto parallelizer recognizes the opportunity to parallelize. You didn't say whether you turned on -par-report and -openmp-report to see if the same loops report parallelization. In my test cases, the current compilers, with normal options, are quite reluctant to auto-parallelize. Auto-parallel never was advocated as a replacement for OpenMP.
The 10.0 compilers were much more aggressive with -parallel than the recent ones, leading to justified complaints about the priorities.
