When scores and grades are in range, it depends on :
-how much ophtho research you already have or can have without the research year. Some research-heavy schools already a lot a significant amount of dedicated time for research (often done by shortening pre-clinical curriculum to 1 or 1.5 years) and in those cases it may not be the best use of time if you can already get enough research without the research year.
-your school's reputation. If you're a Caribbean grad it might not help much as your chances are probably low either way. It can help especially for those at lower tier US MD schools that don't have an in-house ophtho residency and you go to an outside big name research institution to do your research year.
A research year can hurt a residency app if:
-you're not productive during your research year, for example your projects not working out so there's little to show for it
-something doesn't work out well with your PI (it's probably a red flag on residency apps if you full research year but don't have a strong research LOR from your PI)
-Ophtho match may become significantly more competitive the year your apply compared to the year before (though this is obviously hard to predict). There's already a tendency for "score creep" (ie Step scores tend to increase over time) and this can affect students who take significant time off (eg MD/Phd) but already had borderline lower scores to begin with.
