From my point of view, I think it's a harmless change that makes the code better readable, safer and faster (even though the performance gain here has a negligible impact). I don't see a reason to not merge it.
So now we have readability, memory safety and performance on the plus side. To further back my performance claim,  investigates the difference between stack and heap allocation. Short answer: depends on the platform, but stack allocation can easily be faster by two orders of magnitude for a simple single-threaded program. KMail is multithreaded, so we can expect bigger differences: Heap allocation causes threads to synchronize on most platforms, because the heap is a shared resource (see e.g. ). The kind of allocation also hints at the expected lifetime of an object (in C++; in Qt, this is somewhat questionable).
Note however that the performance claims don't apply here because menu.exec() is a "time bottleneck", if you call this function 100000 times you can't say it ran noticeably faster, since all the time was spent in exec().
anyway, +1, it's good to have code without new and delete