I have just had the SSL kit fitted to my Plus Six... what a transformation.
Gone is the wallowy feeling when you go into a fast corner and the front-end used to kind of tip into it, you never really knew whether it was going to grip or understeer and the rear didn't give any clues that it was actually connected to the front!
The car is now a little stiffer but not uncomfortably so, it is confidence inspiring when pressing on enthusiastically and it feels like a proper sportscar rather than a cruiser.
If you are thinking about having it done, just do it, you won't regret it.