Leave it as it is if the engine is happy.
The rear lambda is only for the EML light. It's part of legislation that emissions systems must self check. It just looks for extra oxygen in the exhaust stream which is a sign the cats aren't doing their job. But since the rear lambda is behind both cats, it doesn't know which one, so just bungs the Money light on. It doesn't alter anything in the tune since it can't determine if the extra oxygen is coming from a lean mix or a dead cat. That is the role of the front lambda sensor and is the one the ECU tunes itself from.
Those spacers move the rear O2 probe's tip out of the exhaust stream, thereby fooling it into reading the correct CO2 levels.
