I'm not that worried. The amount of real content in each book is substantial, and the show chops so much down. Take the Sand Snakes or the Kings Moot. Both are INFINITELY more interesting then what is in the show.
I think that the writing of the final book will be quicker, since it should have a lot of the plot heading in the final direction. I also understand that might be wishful thinking since he will probably rethink every chapter twice.
I think the book and show will have different endings. They wont be drastically different, but George will focus on making sure their are differences that are worth discussing.