The Steam version of Shift 2 Unleashed has an issue in multiplayer where unless you're playing with other Steam users although you can see and connect to retail and Origin users when you race online with them you will see each other bouncing around like this:

http://www.youtube.com/watch?v=wnML3ayhLLI
Basically although the Steam version is fully patched something is different compared to the fully patched retail and Origin versions of the game and it causes this issue to occur, to fix it you need to do the following which I have tested myself and can confirm it now works fine.

Step 1: You need certain files from a fully patched retail or Origin version of the game, you can also put the key from the Steam version of Shift 2 Unleashed into Origin and download it through there if you wish (this is how I did it) or you can find and download the files needed online.

Step 2: You can skip this step if you have chosen to find and download the files needed online. If you own the retail version of the game download the Legends and Speedhunters DLC patch from the Origin website here[www.origin.com] or from here[www.origin.com] if you own the Origin version of the game (both links include patch 1 and 2 as well as the Legends and Speedhunter content) as for some reason the game doesn't automatically update through Origin.

Step 3: You need to replace the following files from the Steam version (located in steamapps, common, Need For Speed Shift 2) with the equivalent retail, Origin or downloaded files:

Need For Speed Shift 2/Pakfiles/Vehicles/Koenigsegg_Agera_Cockpit.bff
Need For Speed Shift 2/Pakfiles/Vehicles/Koenigsegg_Agera.bff
Need For Speed Shift 2/Pakfiles/Vehicles/Alfa_Romeo_8C_Spider_KIT03.bff
Need For Speed Shift 2/Pakfiles/Vehicles/Alfa_Romeo_8C_Spider.bff
Need For Speed Shift 2/SHIFT2U.exe

Step 4: Delete the following files from the Steam version Pakfiles folder as they aren't present in the retail and Origin patched versions and were supposed to have been removed when the Steam version was updated:

Need For Speed Shift 2/Pakfiles/IDEMOGPHASEACTIVATE.bff
Need For Speed Shift 2/Pakfiles/DemoMenuTextures.bff
Need For Speed Shift 2/Pakfiles/DemoMenuSetup.bff

That's it, you'll no longer see people who are using the retail or Origin version of the game bouncing around in multiplayer and vice versa and you can still connect with other Steam players. Also doing this doesn't disable the Steam overlay and if you chose to do this by installing the retail or Origin version of the game then you can uninstall that version as it's no longer needed.