Assuming the same offset of 38mm, 20x9 is a much bigger wheel and tyre combination than 17x8. Don't forget, it's not just greater diameter being 20", but it's also a an inch wider at 9". This means the rear tyres are going to extend by 46mm more than the stock AP1 setup. Add lowering and you're going to need new rear-fenders/fender extensions.
Whatever rocks your boat, but I can't see why you'd want to stick 20" wheels on an S2000; you'll completely destroy what the car was made to do.