How rare is it that recordings are sped up or slowed down after recording, by a few percent, during the mastering process?
Not that rare. The producer felt a little extra energy could be gained by raising the speed and/or pitch a little. Or there may be more pragmatic reasons, making changes to fit in with the pitch or speed of what precedes or follows.
This used to be common before the days of digital - often the record company would complain to the producer that a song is too fast or too slow and the option would be to vary the tape speed on the master.