I tried an "experiment"--instead of an HDMI out from the 8100, I used a DVI to HDMI. As soon as I plug that DVI to HDMI into the XPS One HDMI, the 8100 screen resolution reads this, changes from 1920X1200 to 1920X1080.
Whether I use HDMI to HDMI or DVI to HDMI, same result.
So, somehow I probably need to configure the ATI Radeon (in the 8100) through its Catalyst Control Center to output the full 1920X1200 (and not 1920X1080). That would fix things, but how to make this happen is still unknown to me?