It's not as simple as saying "playing a high line" though. When the ball is lost, the backline will push up or drop depending on the level of pressure on the ball. If there is no-one pressing, the line will drop 5 yards, regardless of what Rodgers "prefers" to do. On the other hand, when the ball is won, the backline will drop and split, to create depth and support, in order to open space in the middle. This is the key to the possession game. Think of it like an accordian, expanding when we have the ball, contracting when we don't.
As for the OP, I think an impact sub is someone who is going to score goals from the bench, and as of now, we only really have Bellamy who can do that with any frequency. The ability to get goals, I think, is going to be a key factor in who Rodgers brings in (hence the Dempsey connection, Borini, and whoever else he buys, regardless of position - an extreme possession game like the one he wants to play needs game-changers like Messi and Villa and Iniesta, or it needs the goals to be spread throughout at least 8 of the 10 outfield players).