Obviously a not Relic tank is going to have a harder time keeping aggro against a +1 Relic. I've done Titan HM myself plenty and unless the Whm starts spamming Medica II during the early phases (like M2 for first stomp, seriously?) I've never seen the tank have issues holding aggro. The problem comes when you spam Medica II way too early causing massive overheal on the entire party.