Once you get over 18 inches the weight of the wheel and the low profile of the tire really kill speed, efficiency, ride, and handling. There is no reason to put wheels larger than that on the car. It just makes it look ghetto. But I agree with hftech, I'd hate to see the Camaro suffer the same fate as other modern performance cars, where the "upgrades" are massive chrome wheels, darkest tinting possible on all windows, and blacked out lights. What logic is there behind thinking these kinds of things look good? But anyway this is all
Back on topic, I can see convincing arguments for both sides. Historically, the Z28 should be the all out race machine. It might not get the biggest engine, but it should be the fastest around the track, and should even be able to hold its own in a speed contest against some of the huge engine customs done by third parties. But if you're speaking in modern terms, SS tends to signify the top trim level. Really, I think the Z28 should be the lightest V8 trim, possibly even lighter than the base V6 trim with an emphasis on handling and pure racetrack speed. SS can have the monster engine, but also more comfort, more of a grand touring type car. Which one is the top trim depends on what you are looking for. Ideally the Z28 should be so hardcore that most buyers won't even use it as a daily driver, while the SS can be used as a daily driver and still be pretty fast. This is just my wish list and in no way is a likely guess at reality, but I'd have the Z28 constructed with every weight saving possibility (aluminum frame, carbon fiber panels, AC, radio, etc delete options, lightweight race wheels, possibly even a rear seat delete) with a high-revving but small and light V8, summer tires, and a very aggressive suspension tuning. On the other hand, the SS should have a beast of a motor and sporty suspension, along with such luxuries as magnetic selective ride control (switch between cruising and sport) leather seats, auto-AC, etc.