He'd be a great keeper for Sunderland, but I hope he stays. Anything can happen in football.He would still need to demonstrate his ability to do it under pressure in a match situation. A big part of the skill is the decision making process. being able to size up the situation decide what the best options are. Sometimes Row Z might actually be the correct option. I don't think he can hone his judgement just in training.
He would be under pressure at Sunderland, but its the type of pressure that suits him. His best games have always been when City were under the cosh eg Barca, Dortmund, Spurs. He struggles when he's only called on to make a couple of saves in a match. If he goes to Sunderland I guarantee that within a few weeks he'll have had a superlative game, making a string of amazing saves. I can already hear Alan Shearer on MOTD "Why on earth did City let him go?"
I suspect that Guardiola has wanted Bravo and ter Stegen from the outset, and why not. He might have told Txiki to go out and set a deal up, and then if his fears about Hart were confirmed, then said go ahead.