You'll notice a big difference. Your t-bars will be going up to around the 300 lb/inch rate, you would be adding a rear sway bar with the softer rear spring rate, and most significantly,correctly valved shocks to match it all.

IMO, running 15" tires isn't the big detriment a lot of people think it is either. Granted, the choices are thinning out, considerably, but there are still decent gripping, speed rated rubber out there for 15" wheels.