I went ahead and bit the bullet and installed the NEC V20 CPU. I lose some compatibility, but gain some speed. I'm keeping the 8088 in case I decide to go back.
There's a version of the XT-IDE ROM for 80186 CPU's. I'm wondering how much performance improvement I'll get if I update my card to use that version. But I'll have to remember to revert back should I go back to the 8088, so maybe I leave well enough alone.