I think he can improve his outside shot with work, but I also still think the physics of his body play a role in making hard for him to have perfect form.
I actually think that one of the problems may lie in his trying to develop text book form. I just think those hands are too big for him to be able to rest the ball on his finger tips out in front with a perfect 90 degree angle at the elbow. He compensates for that by narrowing the angle on the elbow and bringing the ball back towards his ear so he can get the right flip.
I think he might be better served bringing the ball up above his head a bit. This might make it easier for him to get a good angle on the elbow and still be able to control the ball with the finger tips and not the palm.
I'm no Juergen Gershwinder (or whatever Dirk's private coach's name is), but I feel like it might be time for Rajon to try something a little more unorthodox and make it work for him.
He could take a page out of the books of some NBA notables including Magic Johnson, Purvis Short, Gary Payton, Jamaal Wilkes (oops), and Mark jackson among others who didn't have the prettiest of strokes but were effective shooters.