Do you plan to include a Gentoo box in the multi-distro comparision? It would be interesting to see the effect of optimizing for the local processor with -march=native.
Though essentially Gentoo is not a single distro. Off the top of my hat I would guess that at (almost) full-system LTO, march=native and -O2 vs. -O3 might make a difference. And naturally the desktop environment… (on a stock Gentoo you don’t necessarily have X…).