It's hard to say really. He clearly has the talent for it, but definitely lacks the killer instinct. The hot shots and flashy winners were acceptable and understandable when he was 18, 19, and even in his early 20s, but he's going to turn 24 this year and he's still trying way too hard to be a crowd pleaser.
He's improved his fitness immensely, it's staggering how much better his stamina is than it was a year or two ago, and he can defend better than most. In my mind there are 3 things Grigor needs before I can see him winning a slam:
1) Improve that topspin backhand, right now it's clearly a liability.
2) Better court positioning and strategy. Stop standing so far back and playing defense, it keeps getting him into early-round 5-set wars of attrition in slams.
3) Cure the mild case of Monfils-itis.
1 and 2 I believe could be solved with a change of coach. Dimitrov's backhand has regressed under Rasheed, who I've never really been sold on. He needs a coach who will cater more towards his offensive capabilities, perhaps a Magnus Norman or a Darren Cahill.
#3 is going to be the difference-maker, because that tends to be a character trait and not a skill that can be easily improved. I'd say odds are still better than 50/50 at this point, but he really needs to start addressing his weaknesses with more urgency or else his chances are going to start plummeting.