How can a player have good mobility if they can't run up and down the floor on a fast break? Grant has OK lateral quickness, but for a player that's 6'5"/235lbs, he has poor mobility overall. He can't jump or run the floor well, which is part of being mobile. If Grant could move better, he would be like PJ Tucker. He is not though, as their athletic abilities are night and day.
You're inventing things now
Which part do you disagree with?
Most of it, honestly.
Grant can't jump. Aside from that, his mobility isn't a problem. I don't see this inability to run the floor that you do. He gets up and down the court just fine. And I don't know what you see when you watch PJ Tucker, because athleticism isn't a highlight of his game either. They're both similarly ground-bound players with good strength and lateral quickness.
I think Grant's impact is already comparable to that of PJ Tucker. The only real caveat to that statement is that PJ is a mediocre rebounder whereas Grant is a bad one. Aside from that, they have similar offensive and defensive roles, and similar performance within those roles. Perhaps I'd say that PJ is more experienced and therefore more aware of his own limitations (particularly with the ball in his hands) at this point in his career, whereas Grant still tries to do too much sometimes. But I'd expect Grant to improve at that, either by continuing to hone his craft or by simply learning where his boundaries are. Regardless, there's certainly no "night and day" difference when comparing the two players.