Intro:
First of all this is the "worst" SLI Fix I've done so far, sry for that!
I tried so many different SLI bits... this is the one which had the best total results.
=> as Racing Games have to be perfectly smooth @60fps... I needed just a little bit more performance for a steady 60fps+ with VSync enabled, and I finally managed it.

Settings:
1920x1080 | 2xMSAA
FullScreen @60Hz
Wait for VSync = OFF
Nvidia Control Panel: VSync = ON (not in the comparison screens)
--------------------------------------------------------------------
GPU Scaling:
Before: GPU1= 93%; GPU2= 7%
After: GPU1= 88%*; GPU2= 88%*
(* varies, see comparison shots)
--------------------------------------------------------------------
SLI compatibility bits:
0x42C80005 (Elder Scrolls V: Skyrim)

0. Download [www.nvidia.com] & install the latest NVIDIA Driver
1. Download [www.guru3d.com] & open NVIDIA Inspector
(note: also extract "CustomSettingNames_en-EN.xml")
2. Go to Driver Profile Settings

=> Set Global Driver Profile to WRC 4

2.1 Then set SLI compatibility bits to 0x42C80005 (Elder Scrolls V: Skyrim)
2.2 & set all other settings as shown below... => Apply changes
3. Enjoy WRC 4 with SLI enabled

Unfortunately after installing a new Nvidia Driver, you might have to set your SLI Profile again!
(Even with the Express Installation...)
=> BUT you can "prevent" that by Exporting all customized profiles and Import them back after the new Nvidia Driver is installed!

i5 2500K @4500MHz
GTX 460 SLI @840MHz / 2000MHz
16GB DDR3 @1600MHz
120GB SSD
Win 8.1 Pro 64bit

Tested on the following NVIDIA Drivers:

• 337.88 WHQL
  
• 340.43 Beta (2nd monitor produced win8.1 bluescreen)
  
• 340.52 WHQL

Standard Nvidia Settings @106,9fps
SLI Fix Profile @196,0fps
--------------------------------------------------------------------
Standard Nvidia Settings @75,8fps GPU scaling: 99% in total
SLI Fix Profile @102,0fps GPU scaling: 133% in total
--------------------------------------------------------------------
It is really not much, but 133% vs. 99% GPU scaling was in my case enough to have a smooth&constant picture @60fps withSweetFX+2xMSAA+VSync@NVIDIA enabled... (see below)
--------------------------------------------------------------------
PLZ Rate if you like... :)

& follow me for more SLI Fix Guides => click