It's all about the GAL in u44. It defines which blocks of memory are addressable. So, even though 384K additional RAM is present on the motherboard, only D000 thru EFFF are listed as addressable by the custom GAL in u44. Therefore, no conflicts with adapter roms or bios rom exists since they normally don't reside in this area.
I'm the one who convinced Chuck(G) to help with the programming of the GAL. Then I bought his programmer. So, if you're interested, I'd be happy to supply a programmed GAL if you want to do this mod. I'd also be glad to assist with setting up the drivers that I used to set up UMB's on my 5155(Use!umbs.sys, Clearmem.sys by Chuck, Dosmax.exe). I have a V-20 in my machine, so i'm not sure if the drivers that I use would work on a 8088. But, the V-20 is a cheap and worth while upgrade for any XT.
edit; I originally typed u84, it should have been u44