Yup. Key to Pep's style is what happens after a turn over - if we cannot get the ball back quickly w'e're going to be very vulnerable.
And yes, likely we'll concede most of our goals on set pieces once we start playing well.
But against Sunderland, our offense wasn't really in sync yet. The buildup was ponderous with players unsure of where to move or to pass the ball. Also we were missing Sane, Gundogan and Kompany and de Bruyne, Silva and Aguero looked tired.
Bottom line - our offense will improve tremendously and once we get a suitable keeper, out ball retention will be even higher (probably not 78% as against Sunderland but very high nonetheless).
Our system worked against Sunderland - we dominated possession, they had few chances, and in spite of our ponderous build-up and relative lack of chances created, we deservedly won.
I don't think that this is a "it works in the Spanish league but not in the EPL" system by any stretch.
Where I think we'll struggle is against teams like Spurs or Pool that press, press, press. Any mistake we make, they get the ball, and they have the pace to punish on the counter.