That specific data would be difficult to track down. You can look at MSAR and it does state the percent of marticulants with graduate degree, but that does not give you an idea whether grad degree holders are any more competitive than the average applicant .
If your grad degree is in sciences or research, you might consider that extensive research experience. I reckon the top research schools might be interested in that. So one could reason that you might be more attractive to top schools (given that you match their stats).
If you are talking about Masters degrees, than the wisdom around SDN is that Masters does not benefit you enormously. However, I have my doubts about this line of thought.