As a Barca fan, I would buy Shaw only if you can get him cheap, which I doubt. Barca are struggling financialy right now and getting another left back doesn't make that many sense imo. Shaw is yound and has potential but has just came back from a tough injury. Barca has Jordi Alba who is playing exceptionally well and Digne has potential to replace him eventually. So unless you would sell Digne, I would rather get a defensive midfielder or another centre back.
Anyway great post and best regards!