I think the better question to ask is, what kind of memory? Research I cannot conveniently find at the moment shows that raw declarative memory is improved under trauma. For example, most people who were old enough, nationwide, can recall where they were and what they were doing on 9/11. Other kinds of memory, like skill automating (e.g. playing the chords effortlessly on a guitar or mastering the core grammar of a language) are much harder to do when you're under stress.