Inserting an offset in the MCDU does not affect its Nav accuracy. The aircraft position is still computed the same way, and the SLOP does not affect the bias in any way.

99% of 320 pilots will operate their whole career under GPS Primary, but in case of loss of RAIM or a GPS block (like military ops), or if you operate a 320 without GPS receivers like a few at my previous company, it is still possible to have High Nav Accuracy as long as the current RNP value is greater than the current Estimated position error (both easily monitored on the bottom line of the progress page). Even then, an offset will not affect the mix IRS position calculation.

Its only if you have a Nav accuracy downgrade that SLOP would no longer be appropriate, but given the sequence of events that would lead to this situation on an overwater route, you would have bigger problems to worry about.