Not knowing a dang thing about newer 9-5's, I will say that general automotive convention tell me that a car overheats because of some other reason....not because of the radiator. If it were back in the 60's....maybe. These days with electric cooling fans etc, even the nastiest V8 can sit in 100* city traffic with the original radiator and not overheat.
Changing your radiator (unless there was CLEARLY something drastically wrong with it, like it had a hole and leaked all of your coolant) would do nothing. You had something happen where coolant and oil are both present.
MAYBE radiator if it has a built-in oil cooler for the engine oil or automatic trans fluid.
Something let go except your radiator, and the shop replacing it did nothing except take your money.