"Vastly" eh? I seem to recall that LLVM ThinLTO has slight regressions compared to GCC LTO on specCPU but on Google's own applications the superior whole-program devirtualization offered only with ThinLTO is a net win.
As a user, building with thin-lto vs full-lto generally produces pretty similar performance in no small part because a huge amount of effort has gone into making the summaries as effective as possible for key performance needs.
As a compiler developer, especially when developing static analysis warnings rather than optimization passes, the number of cases where I've run into "this would be viable if we had full-lto" has been pretty high.
