Bargnani, Gasol (x2), Nowitzki, Weis, Bradly (ok, not European), Sabonis - this has been the range of comps that we've mostly seen regarding Porzingis. Fail (Weis/Bargnani), Succeed (Gasol/Nowitzki). But I don't think any of these really make good comps.
First, I am not 100% sure I like the idea of Kristaps putting on a lot of weight, and turning into a talented low-post player. Part of his amazing upside is his athleticism (aside from his sweet stroke) - add thirty pounds to him, and I doubt you have much more than a spot-up shooter from long range.
The comp that keeps coming to my mind watching him is Ralph Sampson. Obviously, Sampson came out with much more hype, but he was the same thing - 7'4, can handle the ball, can shoot from distance. Too thin (was pushed around by college Ewing), needed to gain weight, etc - turns out he didn't get pushed around as much as it would have seemed.
Sampson played mainly on the low block because that's what you did with 7'4 guys in the 80's. This was before Nowitzki showed how a 7 footer could dominate a game playing 20 feet from the basket. Sampson didn't gain a lot of weight (I think he played at 250-260 his whole career), and played through injuries. But he is in the hall of fame.
Kristaps is going to have his own path, and I doubt its going to look like any of the 7 footers that have come out of Europe to now.