Tackle efficiency hides the situation where players only attempt high percentage tackles. Like when Niukore played centre this year.
A key defensive stat (not recorded, so you have to compile it yourself) is tackles per minute. It reflects defensive work rate; all the little one percenters that contribute to a strong defence. Paulo's work rate is very low for a middle forward. One of the lowest in the game, in fact. Of course he smashes merkins when they end up in front of him, but he doesn't go looking for work, he's not good at filling space or helping get numbers in the tackle. But we play him for long minutes because he is so important to out attack, especially inside the red zone.
I don't see why we would play a different structure while Paulo is off the field. As for our results during that time, if you thought about it before asking dumb questions you might realise there are plenty of other changes that occur while Paulo is off the field, including other players that we are likely to be missing at the same time (RCG, Nrown) as well as some of the opposition's starting middles or even their dummy half having a rest.