A 30 profile tire isn't going to look good with a 20 inch wheel. you need atleast a 35. A 30 will make the wheel look so small on the STS. If your planning on using a 30 tire you might as well go with a 22
Maybe he just likes to spend money. He needs to keep in mind that potholes love low profile tires AND wheels.Keep your original wheels! I'm sure you'll love your new look but if you absolutely hate the new ride, you could switch back. An upgrade in appearance is most often a downgrade in performance. Get ready for an unstable feel. That mostly depends on the offset of the wheel.