I'm not entirely sure what grass you mean--in Antalus, say, the grass there doesn't do anything really to my FPS.
All my details are high except for texture and physics--that's because I've got a GF3 at stock speeds instead of a Ti500 with faster ram or a GF4 with 128 megs for the textures and I only have a 1.4 Ghz Athlon.
Still, even in Plunge, I don't go below 30 FPS. Bombing run maps, which are the ones I usually play, run fantastically.
Plus, there's still no stuttering
--edit--and if you think I'm copping out for running with normal textures, BAH. I don't have a GF4 and in Instagib, going below 30FPS = death.