Unity builds have been largely supplanted by LTO. They still have uses for build time improvements in one-off builds, as LTO on a non-incremental build is usually slower than the equivalent unity build.
I would expect a little benefit from devirt (but maybe in-TU optimizations are getting that already?), but if a program is pessimized enough, LTO's improvements won't be measurable.
And programs full of pointer-chasing are quite pessimized; highly-OO code is a common example, which includes almost all GUIs, even in C++.
Do you link against a version of the Qt library that provides IR objects?
In any case even with whole program optimization, O would expect that effectively devirtualizing an heavily object oriented application to be very hard.
