Now I don't know if it is something to do with the new version of GZdoom (1.3.17), or the drivers, or just that I didn't set things up in quite the same way as the original benchmark savegames (I just warped to the maps, stood in a similar place and used "kill monsters"), but when going to a couple of the maps that were used in the benchmarks and switching on vid_fps I am getting better frame rates than I submitted to the benchmarks thread.
eg, in the Burghead map I was getting about 100fps versus the 82 in the benchmark and in Overlord I am getting 260-ish versus the 211 in the benchmark. In both cases, that represents about a 20% increase in frames per second which can't be bad.
[edit] Hmm, OK, those figures were taken with "speed" rather than "quality" mode and the wrong fog type. I'm still getting an improvement I think, but not as much as I thought I was. [/edit]