The core can be taller but for it to be thicker then 2.5" the bumper support really has to be cut up and the core has to sit up higher since the lowest point on the bumper is too small for anything thicker. Someone installed a ER Comp(3" core) behind a stock A4 bumper, but there wasn't much left of the bumper.

 

My experience exactly! I did the bumper trimming in 100 little steps to avoid trimming more than necessary. The current level still is a bit tight, the bumper is under a bit of pressure when (pain in the but) the sides need to be snapped into its holder/retainer.

Clearly: 2.5 is the better choice (vs. 2.75).