I’m running Elixir 5’s (had them for nearly a year) on my FS & Elixir R’s (had them for just over a year) on my hardtail. Aside from the bite point adjuster I can’t notice any difference.
I have also taken the 5’s for some more gravity fed work in Italy & they performed well enough. I did try a set of Hope’s while I was there (can’t remember which ones, but they did have a 4 piston caliper), which did perform a little better (but I’d expect a 4 piston brake to perform better than a 2 piston).
My 5’s are setup with a 180mm rear, 203mm front, while my R’s are setup with a 160mm rear and 160mm front (no need for anything more as this is an XC bike).