If I remember correctly, HBO basically said they're forging ahead with finishing the show in 7 seasons regardless of Martin. Now, that might mean a movie (or three) to wrap everything up... but excluding that option for a second:
We know that since books 4/5 cover the same concurrent timespan and that HBO isn't going to do a "split" like Martin that it's a possibility that HBO maybe does them both in a "two part" season (like they've done with other shows). So Season 5 might be Season 5, Part 1 (8 episodes) and Season 5, Part 2 (8 episodes) all in 2015. Or, they're simply going to cover 4/5 in two full seasons (the Season 5 and Season 6 we already know they have renewed).
So unless HBO starts doing longer seasons there's no way they get through all 7 books by Season 7. If they did, then Martin would be in trouble as HBO likely hits season 7 no later than fall 2017.
Martin is unlikely to release Winds of Winter this year, because it's Martin. So with a 2015 release he's looking at having only about 2 years to completely his final novel or get lapped by HBO. That's... unlikely to happen.
I think this is all totally correct, and yet ... I just can't imagine that Martin will allow the show to steal his thunder.
Again, according to HBO at least, HBO knows the ending and has no problem going forward with it regardless of which books Martin has finished by the time they are ready to shoot season 7.
I think a lot of people want to watch a show/movie based on a book they've read because they want to see the events described in the book represented visually. That's obvious. Martin readers love the GOT show.
But I don't think as many people want to read a book describing events they've already seen on film. Hardcore fans would, to get more of the backstory, etc., but casual fans aren't going to spend hours upon hours reading a book when they already know the ending.
Plus, if Martin lets the show end first, he almost gives up his claim to being the person who hands down the "authentic" version of the story. If his last book diverges from the show, people may even accuse him of altering the proposed storyline just to differentiate the book from the show people have seen.
I know he doesn't want to rush the last book, but really, it's in his best interest to get it out there. I'm hoping against hope that he will really put his nose to the grindstone on it.