I don't believe there's any data comparing the actual scores. Probably because the tests are administered by different organizations (NBME vs NBOME) who publish data but do not collaborate with each other as far as I know.
There is data on pass rates of the USMLE, comparing US MDs to DOs to IMGs. Interestingly, the most recent data (2007) showed that DOs had a higher pass rate than US MDs on the USMLE Step 2 CK (96% vs 95%). I saw this data in the "First Aid for the USMLE Step 3" book, and it can probably also be found on the NBME or NRMP website. I know it's not a huge difference, but this should squash any lingering biases out there.