It makes a good story,  but it is probably more urban myth.

For all practical purposes HP-UX machines and MPEiX machines are EXACTLY
the same boxes.  On K-boxes the MIO (multifunction I/O) card is a little
different, but not much.  An MPEiX box can and does boot up HP-UX, if you
look at those darn diagnostic DAT's you'll see HP-UX 9.0x all over them.
An HP-UX box can boot up MPEiX if HP comes out and enables the box.  This
change is as simple as change an encoded word in the EEPROM on the system
back plane.   For example,  I've had a back plane in a 979KS400 that had a
problem.  The back plane was replaced,  most of memory and all but one
processor was pulled out and the machine was booted from the diagnostic
tape (HP-UX).   The machine came up as a K360 (?). Another change was made
to update the EEPROM,  we shut it down, added the memory back in, pushed
the processors back in and rebooted again,  the box then came up as a K460
machine.  Again the EEPROM was updated and this time told to be a 979KS400.
 We rebooted again and .... it was an HP e3000.

As for special firmware,  the hardware instruction set is identical for
both boxes,  its the OS that makes a difference.  MPEiX does not have a
full 100% implementation of the HP-UX library calls and shell commands.
The POSIX library is very close, but still not the same as HP-UX.  If I
remember correctly Oracle provides their own "file system" for their
database, while a robust file system is included free as part of MPEiX,  by
file system, I mean something that understands multiple types of files,
records, etc... not just byte streams.

Because Oracle is/was imposing their file system on top of MPEiX, it
probably is somewhat slower then the HP-UX equivalent.  Because the POSIX
shell is not the same as the HP-UX shell, it means that ORACLE scripts
execute somewhat differently.  So the ORACLE DBMS installation scripts are
not exactly the same and probably take more work on the part of the Oracle
company to maintain.

I do remember that at the Orlando database roundtable back in about
1986(?), that Oracle defended itself against questions of speed by stating
something to the effect that it couldn't be as fast as Image, because it
was a "user mode" application and not taking advantage of special operating
system hooks.  However Oracle at that time offered solid a solid SQL
environment and of course the ORACLE development/production environment.
(Kind of like COGNOS stating they are the only one to offer a Powerhouse
environment.)

Comparisons of Oracle on HP-UX and MPEiX for TPC's is probably a dead issue
if Oracle does decertify the HP e3000.

Comparing transaction rates for HP-UX/Oracle and MPEiX/TurboImage would be
interesting, but I'm not sure that the IS V.P. would accept a comparison...
MPEiX has the preconceived reputation as being an obsolete legacy operating
system.  HP's silence on MPEiX when touting Win2000, NT, HP-UX and now
Linux, does little to prove otherwise.  So when comparisons are done, they
typically want TPC's for WinNT, HP-UX, LINUX, Solaris, etc.... but not an
obsolete product..  (read this paragraph carefully, I'm not saying it is
obsolete, I'm saying the perception of many IS VP's is that it is...)