personal experience and research.

ROM revision 13 does make more boost on factory boost control, not many cars had this version as version 15 released shortly after the IX's were released. It's pretty weird because the tables in the ECU are actually all identical so it's probably a code fix that reduced the boost. My IX came with revision 13, I purchased it in Nov of 2005, it would spike to 1.5bar and taper to 1.3 on the stock boost gauge. On my AEM boost gauge it measured a full 21psi stock. Car's mfr'd later come with Rom Revision 15 cars spike to 1.3-1.4 and taper to 1.2bar on the factory gauge.

The first time I tuned one of these later revision cars I thought it had a boost leak or something wrong with the factory boost system. Turns out it's something internal, I'm not sure if the real guru's have identified what was actually changed in the revisions but it's not in the known boost tables. Like I said it's probably a code fix.