My school heavily discourages taking the USMLE, but this may have changed recently due to the merger. Even so, 60% of my school takes it, we have usually a higher or at least equivalent USMLE pass rate as COMLEX pass rate and according to the school admins, the USMLE average was in the high 220s (i.e. approximating the national US MD average).
Also, a lot of weaker students might take it for geographic reasons. There is a clear geographic prevalence of residency programs that are "USMLE only" in some regions vs. others. A lot of stronger students may similarly not take it because they knew they were applying to the competitive AOA programs only.
A lot of my classmates chose not to take it for one of 3 reasons: (1) They bought the claims of the school when it told students that 70% of ACGME residencies "accept" the COMLEX (accept maybe, but view as equivalent definitely not) or they already know they are applying only AOA, (2) They didn't want to pay and spend extra time preparing for 2 sets of exams (these people all talk about regretting their decision come the end of 3rd year), and (3) They are truly weak students that barely pass the COMLEX (<450) to begin with.
If I were to make a guess at how many would pass and what the average would be if all had to take it, I think the vast majority would still pass, maybe more like 85%-90%, and the average would still be in the 220s, but probably the lower 220s. The student population of the average DO school is not that academically different from low/no-tier US MD schools, so it wouldn't be a surprise for the averages to be similar. That said, I'm sure there would be a significant difference between the best DO schools and the worst/newest ones.