EasyJet are actually the first operator to put as many as 156 seats into the A319. Previously only 142 was done (Skyservice Canada and Germanwings). So it's not just another 6 seats but 14 seats beyond the previous limit, presumably determined by evacuation tests, which thus requires the double overwing exits.
As the double doors and surrounding structure are on the A320 anyway, presumably it's just a case of combining standard parts. Anyone got a feel for how much it adds to costs ? The idea probably came as much from Airbus as from Easy during the sales process.