I am personally interested in the code amalgamation technique that SQLite uses[0]. It seems like a free 5-10% performance improvement as is claimed by SQLite folks. Be nice if he addresses it some in one of the sessions.
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.
[0] https://sqlite.org/amalgamation.html