A post-bacc program typically consists of pre-med courses. Are you in an SMP? If you're taking classes alongside med students (e.g. anatomy, biochemistry, etc.), this should help you perform better during your M1 year as you'll be better prepared to handle the volume and pace of medical school coursework compared to your peers fresh out of undergrad. This may result in a better outcome on boards. However, this is largely dependent on the type and rigor of the program you're enrolled in (i.e. 1-year vs. 2-year, courses taken). I wouldn't assume you'll perform well on boards just because you did a post-bacc, but it could certainly make the adjustment to medical school easier! I just graduated from a 2-year SMP and in my opinion, I feel much more prepared to start medical school in the fall than I did 2 years ago. Good luck!