It's a popular vehicle, so there should be a lot of various wheel and tire options that should have the same outside dimensions as the stock 20" setup. There should be no need to tweak the speedo unless you decide to put a lift on it and go oversize.
That's interesting. TireRack.com shows the OEM tire size for the Limited as 255/50-20 and Discount shows 2 OEM sizes as 245/50-20 and 255/50-20.