I also have a Nvidia GTX 550 TI like Pierre-Albert and game is running far too fast for me too.
I'll try Powertools but their FAQ implies that unless it's the Steam encrypted version, it won't work:
>Do you play Saints Row 2 through Steam? If you bought it elsewhere, Powertools currently won't work for you! Contact me and I'll see what I can do for you, if you own the game legally.
Ok, update. Powertools does not work on the GoG version of the game.
Yes, it does. It has been confirmed that the GOG version of the .EXE is exactly the same as the Steam version, just without the encryption. Also, numerous people (including me) have used it and found that it works.
Did you follow the installation instructions presented in the Readme.txt that comes in the Powertools .ZIP? Note that you do have to manually edit the [speed] value in the Powertools.ini (configuration settings) in a text editor such as notepad if you want the speed to be reduced. 1.0 = 100% (which is the default game speed -- way too fast, in other words), 0.6 = 60%, etc. What setting will work best for you will depend on your individual PC, so you'll have to use a bit of trial and error. 0.9 is probably a good value to try for starters, then tweak it downwards until you can home in on what works best (keeping an eye on the timer during races or other timed missions can help you figure out if it's still a little too fast or slow).
Also, be aware that Powertools really only helps with the speed -- there are still numerous other problems with the game (crappy over-sensitive driving controls, for example) that you'll need to find mods or other community fixes for.