PS really tests how well you know the equations, concepts, etc. It's the "easiest" to improve in because you can really master the stuff, unlike VR (lol) and BS, where it's a lot more critical thinking and subjective interpretation of experiments and whatnot. If it's a struggle for you, then that means you're not as familiar with the material as other people are, and understandably so if you've never ever seen EM material.
WRT AP being a booster, it depends on how you learned physics the first time around. Did you just blindly memorize equations and use them with brute force with exams? Did you forget all of the equations the minute after you took the AP exam? If so, it probably won't help as much as it could if you actually studied it seriously and internalized everything. Only you can ask yourself for the honest answer.
Because yes, if you
mastered AP Physics B/C (doesn't matter which) material, MCAT physics is cake. If you went about it as a boring requirement you had to get done and be over with, it won't be as helpful as you think because you'll have forgotten most of it. (Pop quiz: there are at least 2 equations for voltage: can you name them?)
I say this because I did about 4 days of content review, realized it wasn't doing much for me, and stopped. I took sciences really seriously in high school (and ironically put it in the backseat for college), made the semifinals in the Olympiads, 800s on SAT Subject Tests, made sure I knew concepts and wasn't blindly memorizing, etc. So even though I haven't touched physics EM stuff since senior year (didn't even take AP), I could read a question and know what equation it wanted and subsequently pull it out of my ass. That's seriously what it felt like. These were things I hadn't thought about for two full years, but it came back instantly. Afterwards, there were only a few more physics topics I needed more review on, like fluids and electrostatics (lots of equations, easy to confuse them). Chemistry required almost 0 effort, all thanks to USNCO
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Just my thoughts on PS and past experience with sciences helping for the MCAT. I'd go as far to say that anyone who has really mastered (high bar) AP Chemsitry and AP Physics material needs minimal studying for PS. Different schools and teachers hold students to different standards for satisfactory completion of AP at their own schools, but mastery is a different thing. I believe mastery is attained when you can answer a question accurately about a topic you haven't touched for ~2 years within 3-4 minutes of moderate thinking and recalling.
I've seen this sentiment reflected in most of my peers at school as well; it's the BS that gets us