I suspect a true perfectionist (one who's obsessive about it) will complete nothing. But there are two different processes: perfecting the tale and perfecting the telling.
As someone who tampers daily with what I've written (and it doesn't matter whether I've been previously more than satisfied with the particular piece of text**), I'd agree that law of diminishing returns. Apart from anything else, GRRM is writing tomes; he and we cannot expect every sentence to be a wonder and if all GRRM was trying to do was make them so, I'd be a bit miffed if I'd been waiting for the book for five years.
I'm not sure that this is the case here, though. He's struggling with the
tale, not the telling. Once GRRM has put each part of the plot in the public domain, that's it. With key plot points, he and we are going to have to live with them. They're not just sentences that, on reflection, may be a bit ugly, but can soon be forgotten; one misstep with the plot could be a wrong turn on the road to the book's conclusion. And if a wrong turn were to be published, I suspect even greater delays to the subsequent books, because GRRM will be spending lots of his time and effort trying to recover the situation.
There's worse: With a story as complex as ASoIaF has always been, a recovery of that sort might well scupper the whole project, which I for one would deeply regret.
** - I'm no perfectionist: my prose
really needs a good rewrite.