[fielderLS3]

A smaller wheel/tire diameter will lower the height of the car by half of whatever the change in tire diameter is. If you go too far, you may bottom out/scrape driveway entrances, etc. You also want to be careful not to go so small that the wheels don't fit around the brakes. You'll probably have to get the speedo re-calibrated too.

I guess the pros would be lower unsprung weight, lower rotational inertia. However, you can accomplish this with aftermarket 20" wheels.

If you are looking for better gearing, I'd just put in different gears. Probably less that can go wrong, since the ride height, handling, steering, alignment, and other characteristics of the car would be optimized by GM's engineers for the wheel/tire size that come on the car. Besides, gears would have more of an effect (going to 19s would only be the equivalent of going to a 3.63 gear).