The effect of beta blockers is probably very individual. A good friend takes beta blockers to help her concentrate during tests - and she saw vast improvement when she started them. Importantly, she wasn't concentrating due to minor panic attacks, so it may not work in that way for someone who isn't panicking. I'm sure you'll be able to tell in your undergraduate studies whether your studying abilities are going up or down, which won't necessarily tell you how you'll do in medical school, but at least you can find a trend.