i took 3 courses after being in USP for 3 years...I'm not gonna graduate with it, nor will i quit. USP has opportunities (overseas programs, supervisors, accomodation, fundings, flexible corriculum, etc), and challenges that almost offset those, for some.
Joining never hurts as you can quit anytime. However, one thing science students shall note: if plan to do research, esp with a head start, it is not wise at all to read many humanity courses. USP now requires 5. Too many in my opinion. 2 or 3 should be healthy. USP won't let that be compromised, though (believe me I tried).
CAP should not a big worry---1 or 2 B or B- is okay. I got B in writing, but will still maintain a cap between 4.8-4.9 at graduation. In the end, if you go into industry, a 0.2-0.3 diff. in CAP will not be nearly as impt as what I think you can gain from USP. If you go into research. If you go into research, mostly likely you will do grad study in NUS. And again a lower CAP---as long as it's over 4.0---still meets admission criteria.
The point though, is not about CAP...it's about whether the courses you read here are in line with your goals. If you are a highly academic science student, you can join, read a few courses useful, then quit (but without officially quitting).