unless you are a serioustly quick learner or have excellent teachers, the chances of you doing enough braille by touch to sit an exam in a reasonable time seems slim.
however absolutely worth doing for your future, no doubt about it.

Anything over 20 font is basically to big to be efficient Are you an avid reader?.

It won't hurt you to know it, but so very little is brailled that it honestly hasn't come up in my life. When I use Braille it's mainly bc I labeled it myself.

For now, no. You're in your exam years, so you've got more than enough on your plate; if your sight isn't declining to the point that even 72-point will become unreadable in the near future, I would continue to use that for your exams; nothing like focussing on something else to ruin exam prep. Later in life, absolutely. Some things are very hard to do with a computer and audio input (making presentation notes for example.)