How to deal with extra points in exams?
When I have dealt with extra credit in the past, it was simply retained in the records until the very final accounting of scores at the end of the course.  Thus, if the exam was out of 100 points, it was still considered to be out of 100 points, and some people simply had more than the maximum possible.

When grades were combined together in the final accounting at the end of the course, having a score above the maximum effectively ended up helping compensate for lower scores on other tests (though perhaps not one-to-one, depending on relative weighting).  If a particularly amazing student turned out to get more than the maximum in the class overall, then it wouldn't affect anybody at all: the curve wasn't shifted and they simply got top marks, as they would have if they'd gotten the maximum.