a) Did you verify that your memory is still running at the same frequency/timings it was prior to installing the extra 16GB?
b) Depending on what benchmark you're running and how much "a little lower" is, it could be within the margin of error for that test, meaning the two results are essentially the same. Which could make sense given that going beyond 16GB of RAM is useless in the majority of cases.
Edit: No need to create multiple threads about the same issue..http://www.tomshardware.com/answers/id-3582659/adding-ram-slow-performance.html