A proper spacer or elbow or tune will get rid of the catless CEL P0420 and not cause other codes. Tune to do so is the sure bet.
This can be true in some cases regarding tuning it but not always.But tuning it out leaves the sensor in a not ready state. That would usually cause a fail for emissions testing. On one of my previous cars I managed to get a secondary 02 in a ready state with a spacer, but it was temporary. The cell would come up every now and then, but I made sure to schedule my appointment when it was green