If the ECU is reaching its limit stops, then that indicates that
the sensors involved in lean/rich mix are sending signals , but the mix is so far off that the mapping can't compensate.... so I doubt
an 02 sensor prob...

With a vac leak, there is just too much air for the system to correct to... and with a bad regulator, [ w/high pressue reading], there would be too much fuel to correct to..
That's why injector duration times and fuel pressure, along with live stream data , are important to proper diagnosis..
Mixture is not just controlled by the 02 sensor...that is the final Tweak...