RJ can knock down corner 3s at a respectable level. I know he didn't take a lot last season, only attempted 75 corner 3s. He made 27. That is 36%. It must be noted that he shot 39.5% from the right corner and the right corner 2, he shot 57.6%. He adds that extra dimension of pump-faking, and being able to step in two or three feet for his high percentage shot at least from the right side. That was last season.
Going backwards, in Milwaukee (his career scoring year), Jefferson shot 45.8% from the corner 3. In a coincidence, he took and made the same amount of shots from both corners in 2009. From the right 45 degree angle, he shot 43.2% from 3 as opposed to 29.5% from the left 45 degree angle.
My conclusion, though not perfect, is that Jefferson has the ability to shoot both corner 3s but is somewhat weaker on the left side, as opposed to the right. His inability to get shots in due to the fact that he is opposite the floor where Tim Duncan sets up from, the left block. Jefferson has always been placed on the right side of the court to maximize his efficiency and not his touches.