I guess you mean Bob "Guru" Rawle
he squeezed a shade over 400 BHP from my methanol injected bugeye.......legend !!
Bob would have told the ECU to ignore the rear 02 sensor saying that the cat is running below efficiency (the P0420 CEL) but the rear 02 sensor is still monitored by the ECU and if it thinks it's broken or there is a bad circuit connection then it will throw a CEL
It's not that simple !
http://en.wikipedia.org/wiki/Catalytic_converter
If the Front 02 sensor is playing up then the ECU could get confused but I doubt it.
The Bugeye ECU does not have enough room on the EPROM for two completely individual maps but can cope with a switch to a low boost setting independant of the fuel / ignition map (foot to the floor and rear demister LOL)
Unplugging the rear 02 sensor will just let the ECU think it's completely broken..
Any good subaru specialists near you ?
Cheers
Shaun