The DiRT Showdown profile included with the current v30x.xx drivers does not work properly with SLI as it shows poor scaling (25-50 fps in-game @ 1920x1200, Ultra, 8xQCSAA on my system).
It contains what I believe are two errors:
1. It doesn't include the showdown.exe and showdown_avx.exe executables
2. It has the incorrect DX1x SLI flag setting of 0x080100f5 instead of 0x081000f5
This is my profile which you can edit into a profile text file exported from NVIDIA GeForce SLI Profile Tool or use NVIDIA Inspector (the easiest option IMO):
Profile "DiRT Showdown"
ShowOn GeForce
ProfileType Application
Executable "showdown_demo.exe"
Executable "showdown.exe"
Executable "rp6.exe"
Executable "showdown_avx.exe"
Executable "showdown_demo_avx.exe"
Setting ID_0x00a06946 = 0x080100f5
Setting ID_0x00a06946 = 0x081000f5 UserSpecified=true
Setting ID_0x00b82343 = 0x00000002
Setting ID_0x1033cec2 = 0x00000002
Setting ID_0x1033dcd3 = 0x00000004
Setting ID_0x10ecdb82 = 0x00000001
Setting ID_0x701eb457 = 0x2241ab21 InternalSettingFlag=V0
SettingString ID_0x7049c7ec = "웭ꑢ쫐₰" InternalSettingFlag=V0
SettingString ID_0x7051e5f5 = "籭鸙" InternalSettingFlag=V0
SettingString ID_0x705fafec = "웭ꑼ쫑₂쁰₢Υ¢婋籯鸹灰땁厈䃈ޡВ뽉襂" InternalSettingFlag=V0
Setting ID_0x708db8c5 = 0x239978f1 InternalSettingFlag=V0
Setting ID_0x709a1ddf = 0x4b1cd968 InternalSettingFlag=V0
SettingString ID_0x70b5603f = "榻鳈⏹ꢗ" InternalSettingFlag=V0
Setting ID_0x70edb381 = 0x24208b6c InternalSettingFlag=V0
EndProfile
I've tested this in over 8 events and the scaling works better than the default giving me a constant 60 fps with v-sync on using the same settings I posted above. It has also been confirmed to work by two other people, one on the Steam forums and the other on the Codemaster's forum.
Just thought it might save someone some heartache trying to get the game running properly. The game is actually pretty good fun with demolition derby events and an overall light-hearted arcade feel to it. The demo does it no favours really.