Porzingis is better than Kornet/Tillman. That alone is huge, even if KP can only go around 20-25 minutes a game and average maybe half of what he usually does (so 10 PPG and 4-5 RPG?).
I still think his presence alone just changes a lot of things. It'll be harder for opposing teams to lob it or attack the basket with a 7' 3" guy just towering around there, and then on offense his ability to hit 3s and just hover around the perimeter might also affect how Dallas approaches things defensively.
I kind of view KP as a "decoy" this series. He can have some great games too assuming he holds up, but I think he can open up things for others and also "maximize" Horford by letting Horford not play heavy minutes and also opening things up for him too.
Everyone is focused a lot on the Jays, Kyrie and Luka. Rightfully so. But I think ultimately this series will really come down to Horford, Porzingis and even a role player like Hauser knocking down the open looks, because Dallas will have to pick their poison and will probably leave those guys open. On the flip side, we can't let their role players go off too (Washington, DJJ, etc.).