I never seen my RPM below 900 simply because i'm a bad sim, i don't use manual clutch , sorry if my language seem's rude, it's not my natural language.
if you learn how a driver works and how gpu are designed you will understand. DX8 is not even maintained as well as DX9, he's simply not maintained it's a fact, it's why somes olders games don't run on moderns video cards or moderns drivers and we have to alter drivers to make them run.
My cpu is a q6600 watercooled (no temp problem) with 8Gb of memory and a ati 5850, when i test my triple screen with dirt2 (hard gpu usage) i get the same frame rate, it may be whole better with lfs since the gpu usage is less, if lfs is not multithreaded, the problem may be here. it's not driver related since ati makes good drivers now.
it seems that you have not read correctly my request. and no there will be a big performance improvement since video card drivers are focused on dx9 (and dx10).