Hope this helps... but of course, this is just n=1:
I focused my AMCAS personal statement on my motivations for MEDICINE... with a small section on the importance of research in this motivation... and made it clear in the last paragraph that I wanted a career as a physician-scientist... HOWEVER, my personal statement for AMCAS was definitely not heavily research!
Then, the secondaries will come in.... and you will have plenty of space to write all about your research in your MD-PhD essays and make it clear why you are doing MD-PhD. If this is your focus in your AMCAS essay, I fear you may become redundant in your MD-PhD essay... and I believe that the most important thing about these essays is to give the Adcoms the greatest insight into who you are without meeting them... why would you want to waste space saying the same things twice??
Hope this helps some.