I'm not changing any more settings in Doom 3, last time I did that I changed the values for:

image_useCache
image_cacheMegs
image_cacheMinK

Which is suppose to give to performance boost according to some people. Well for me it caused severe pauses and stuttering throughout the game (especially when you open doors to enter new areas for the first time), and artifacting in the outside sections. I had to reinstall Doom 3 coz I couldn't remember what the previous settings were, seemed to fix the problem.

Well IMHO John Carmack is a 3D engine programming genuise who has already set all the optimal settings that can't be set in the game nenu, so leave them alone. Any changes you do make will come at a price, just maybe you won't notice them straight away.