The A319s are gradually being replaced with brand new A320 CEO and NEO from next year and will feature an additional 6 seats and the Spaceflex rear galley.
U2 have had the A319 for well over a decade. If there was a way to fit additional seats into the A319 then I'm sure they would have figured out a way to do it by now. Removing, combing or moving toilets, galleys or anything just wouldn't work. I'm surprised it's certified for 160. Maybe Airbus had ideas back when it was ceritified?
Four more seats doesn't sound a lot but with half a dozen flights every day multiplied by 364 then that's a lot of money.