An 8-bit memory bus means half as many RAM chips and buffers. Keep in mind
that the IBM 5150 was intentionally crippled to ensure it didn't compete with
IBM's low-end minis.
Did the 5150 have a UNIX available anywhere near its launch date? I know that it had DOS, CP/M-86, and the UCSD p-System relatively early on. It's not clear to me whether Xenix ever supported the original PC; were there other early porting efforts?
-Henry