I found two articles about QStringLiteral: https://woboq.com/blog/qstringliteral.html and http://blog.qt.io/blog/2014/06/13/qt-weekly-13-qstringliteral/. According to them there is binary overhead. For plain literals and QLatin1String, compilers try to consolidate identical literals so that they are not duplicated. For QStringLiteral, identical strings cannot be merged. It would be nice to have a check that can warn about this overhead. I am not sure which level to use to search duplicates (unit or whole project), but probably it should be possible at least mimic compiler optimization and warn about such cases.
Good idea, but this is out of scope for clazy, as it operates on a translation-unit level. Could be a job for a separate tool, like a linker plugin though