Right, here are a few results from tests I've run on current MAME and the Apricot Xi.
According to http://www.old-computers.com/museum/computer.asp?c=500
the Xi came in a few different flavours, differing in the amount of RAM, hard disk space and the type of floppy drive used (single sided vs. double sided).
My machine has 256KB RAM, a 5MB harddisk and the single sided, 315K disk drive. Its label reads Sony OA-D31V-94.
MAME's Apricot Xi emulation doesn't show the right amount of free space for the two disk drive varieties it offers in the slot menu:
For D31V (which should be single sided, 315K), it shows 630K ... but double sided drives like the D32W were actually 720K ... MAME shows 1440K
My Xi couldn't boot from the disk I created the image from that Duke modified to automatically start the birds.exe game. I had to create a bootable disk from the 70 track, single sided disk found here.
Unfortunately it appears that some graphics or font file is missing - on the other hand that means that MAME's graphics are correct.