Personally, I would not recommend taking organic chemistry during the summer. Having previously taken organic chemistry (and biochemistry) during the last academic year, I believe that the content of these classes are difficult to retain in the long run (making the material learned in a short frame of time even more difficult to hold onto). In addition to the difficulty in retaining and learning of the material (which is also important for the MCAT), as I've read many times on this forum, if you believe that you would be unable to do well in this class, are financially well-off, and would not receive a mark on your transcript for withdrawing, withdrawing could potentially be an option. However, be aware that these classes that are considered to be "hard" in college are ultimately lacking in difficulty compared to the content learned or classes taken in medical school.
In my humble opinion, I believe that taking the course P/F may be a risky gamble in that retaking the class reduces room for a different experience that you could possibly partake in.
@Goro is outstanding when it comes to things like this.
*these are my personal thoughts and they may differ from those that read them