Oh David, have you ever tried the quake map reskins? It makes all the maps smoothed out and improves performance.
Actually I use highframes, so I rarely dip below 60fps. The problem's not so much low fps for me as it is finding a stable one. Kinda impossible since fps seems directly proportional to how much spam's going on at your location.
Yeah, dustbowl isn't good for laggy clients. Also, highframes only maximizes your frames partially, use maxframes for the best frames you can get. Or look into these http://teamfortress.tv/forum/thread/7639-fps-improvement-compilation