Another progress update! The 5155 now has the latest BIOS! It seems to have made the start-up memory check a bit faster which is nice because it took a looong time to count up all 640kb.
I've put a network card with a boot ROM socket with XT-IDE on it for testing. Initially I couldn't make it work but I since found that I left the 8087 copro switch on when there wasn't one in there.
Amiga 4000: Apollo Turbo 040, 128MB Fast, 40gb HDD, CD-ROM (needs internet)
C64: 1541U-II, MixSID, WiFi, LumaFix64
ZX Spec, A1200, CD32, MacPlus/SE, A500, BBC Turbo Master (NASA), 2600, Acorn 4000, VIC-20, IBM 5155, GameBoy