Making it a 1 time use ability sounds like it would be more balanced since otherwise it would grow quite large, but then you should probably keep it at +1|+1 and reduce the casting and ability costs. Otherwise, with +2|+2, popping it mid-game could be extremely powerful (e.g., with 7 pillars, it gets +14|+14). Reducing the cost would also force you to be strategic, making you consider the advantages of playing it / using the ability later for a potentially larger bonus.