Eaton blower, forged internals, T56 tranny, heavier clutch, beefed up rear end, but I am not sure how much stronger it is than the '01, bigger tires and rims, better seats, although the '01 seats are nice. Revised hood, front facia and rear bumper and spoiler plus a flush rocker panel.
If you want to build something to race around a road course, the '03 is not as good of a choice as the '99/'01. Almost 200# over the front end and a higher center of gravity with the Eaton/intercooler on top.
If you want a cruiser that can get out and run, go with the '03. To me, it is a great long distance road car. It is way faster all of the way across the board. You can go to the strip and run in the 12's right from the factory.