As for installing KDE vs mesa git, you're comparing it unfairly. Of course it's easier to install the whole KDE binary packages from the repositories than fetching the ddx, mesa, drm and (possibly) a new kernel and compile them. But what about KDE from source? I took a look at their 'documentation' and didn't even know where to start. The Kwin developer himself stated that it's not a trivial task, taking him over 1 day to compile. Even if I wanted, I couldn't test new KDE versions at all. Although I never had to compile the whole X server to test new drivers, I'd feel much more comfortable doing so than tackling KDE.