@neconnn - yes you can restore the pre SP3 backup; however during the mounting of the database on the SP3 server, we will upgrade the schema of the restored database. It should be seamless however be aware of it as then you cannot take this database and mount it on an non SP3 server.
@pesos, @gerard, @whybothertesting - while I understand where you are coming from, it is assumed that - SP3, being a large update, will take some time to test and deploy in production environments on it's own, whether it provides E2013 coexistence or not. As another commented mentions - not everyone needs to coexistence piece anyway. CU1 is needed and we hear that, and will deliver it soon.