<p>If you truly mastered the material in books like Sakurai, it is truly impressive. I know MIT undergrads who struggle with concepts in that book. I think at that level, you can even start engaging in small personal projects (you might want a mentor to guide you, maybe try RSI?). Just think of them as problems, except more involved and difficult.</p>
<p>Olympiad is fun, and definitely worth trying. If you master E&M and mechanics at the level of Griffiths & Kleppner/Kolenkow, the olympiad problems will be a lot less of a challenge.</p>