Are you sure? The Performa 550 is based on the LC III architecture, which is most certainly capable of doing 24 bit addressing.

I just realised - you used version 1.1, which is a very old version of the Apple IIe Card software, most certainly pre-dates both System 7.1 and the Performa 550. (and the LC III, which it is based off). You may want to try Version 2.2.1.