Actually, if you had a chance to see him live at summer league, he runs the floor very well, and has decent lateral quickness. I thought he showed more potential defensively than he did offensively. He blocked some shots and rebounded well. Offensively he was like a bomber surrounded by jet fighters. Everything was moving way too fast for him. He needs to lose about 20 or so pounds and get into top NBA shape. It would be in his interest to spend most of the year in Reno where he can get serious minutes.
I question his lateral quickness a lot. The reason why I think he has more potential on offense is because he has pretty nice touch around the rim. His post game is a work in progress, but it looks like he can use either shoulder. He didn't take a lot of mid ranges, but I think that'll be a big part of his game moving forward.
Defense will be harder for him to learn. As you said, the NBA is much faster. Lots of different concepts he'll have to grasp. He'll have to adjust his body to the game. He'll be called for a lot of "touch" fouls. To become a good defensive C you need: Size, length, quickness, and IQ(defensive).
Just can't see a high upside for Papa on defense even if he loses weight and bulks up. But then again, Gasol has become an elite defender. It'll be an uphill climb though.
Poeltl constantly pops up in my mind. I wonder if he was a real option for Vlade at 8. He's everything we want Papa to be 5 years from now...