In the 2011 keynote of QuakeCon, Carmack actually touches on a few of the techniques they used to make the play and framerate smoother (at a very high level, but its enough to get an insight), its quite a long talk however, but IIRC he gets into the rage stuff about halfway through (some of it does involve driver hacks courtesy of AMD and nVidia).

In terms of the texture stuff they do (as mentioned above), this thesis goes very indepth about the optimizations you can perform (its quite old now though, so it excludes newer stuff like AMD's PRT's, but its a great starting point).