[decooney]

Paul,
For my car I will stick my neck out to say (with no test proof) at least the body becomes stiffer, but I would not be one to state the chassis becomes stiffer.

One interesting test is to see if you can jack your car up from one point and lift three wheels off the ground. With the car up, then try to open and close the doors on your car and see if they bind up or not. Don't know too many bolt on body's that pass this test very well. Would be interesting to see who's tried it.