I'm much more inclined to say that Ibaka is making a difference because of Perk.
Agreed.
I don't think this is a stretch of an argument at all, and it's one that I've made myself.
The Perk trade freed Ibaka up. Rather than having to defend the best opposing big man, he is now free to "roam" and gamble more, knowing that Perk has his back. Perk's boxing out also leaves more available rebounds, which Ibaka has benefited from. However, the biggest factor has been giving him more defensive freedom.
I don't know who said this -- maybe it was Perk -- but to have a great defense, your big men need to work in tandem to cover for one another. The Perk / KG combination worked very well, because Perk could defend the lane, while KG could roam and help and harass, and could feel free to disrupt on the perimeter. Right now, Ibaka is like a mini-KG, playing the same role.