I doubt they will break unless you smash into a curb at 40 MPH
but I"m sure the bonnevilles will too at that speed. Grand Prix is a lighter car, but it still can be loaded up to be just as heavy as our car ( i.e. big passengers ) so for normal driving and such.. I wouldn't worry about it. I doubt GM would put rims on cars that are at the exact weight that the rims will carry.. they know that there will be times that it gets overloaded and the rims MUST hold up to it, or there will be major lawsuits. If you're concerned about breakage.. then a steel rims would be a good bet.. but since you're not curb hopping police officer, the Grand Prix rims should be fine. I believe they has the same bolt pattern as the bonnie 5 x 115mm. That would be something of more of a concern than breakage.
My 1.7 cents ( damm taxes )