You mentioned that you like sport mode for the stiffer suspension. Does that mean that you have the Dynamic Handling package? If so, you can configure sport mode to affect chassis only (what you want), drivetrain only, or both. When you select Sport mode, "Configure Sport Mode" would be an option if you had DH. If not, then you don't have DH, but that also means that Sport mode only affects steering, transmission, and throttle response, not suspension.
This is one of the main reasons we got DHP. Although to be honest, steering effort is more or less indistinguishable at freeway speeds, as opposed to lower speeds where I can feel some difference. They are all too light though.