level of science curriculum
I think the chief virtue of the science/math curriculum is not the level of the material covered (although special and general relativity, partial diff. eqs., and non-Euclidean geometry all crop up eventually) but the manner in which they are treated. I think the experience of working through Euclid's Elements or Newton's Principia is something wholly alien to just studying geometry or calculus in a text-book. If one's principal interest is studying as much math or science as one can so to have done the hardest topics in math and science, then St. John's is probably not the best choice. Studying the original works (with all the quirks of notation and approach not sucked out in a cohesive text-book) provokes questions about math and science that would never be touched in a traditional math or science class. Having taken a few semesters worth off math classes at UPenn in summers between going to St. John's, I am a big believer in how math and science are treated at St. John's.
To answer your concern about a career in science after St. John's, it is not ready-made but by no means impossible. I myself am starting graduate school next fall in the History of Science (which isn't science exactly) but I do know others who have gone into mathematical modeling, biology, medicine, physics, astrophysics and other disciplines. If you think that a career in science is a possibility for you, taking advantage of summers with internships or outside coursework can certainly make up for some of the difficulties the St. John's education makes for going into science. I believe that St. John's prepares you splendidly for a career in almost anything; the trick is convincing others (i.e. graduate schools and employers) that it does. Because of the novelty and relative obscurity of St. John's, grad. schools and employers might be hesitant to take you into their ranks.