Has the value of a five-card suit in no trump contracts been demonstrated?
I will start by saying that a 26-27 hcp requirement for games is old-fashioned in the extreme. I most often see a recommendation of 25 hcp for 3NT or 4M games, and occasionally see that shaded to 24 hcp. Along those lines, I raise a 15-17 1NT to game with nearly any 9-count.

Probably more useful than upgrading 5332 hands is downgrading 4333 hands. Many players subtract 1 high-card point for 4333 shape. 4432 hands play much better than 4333 hands, and so don't deserve the same treatment.

With all that being said, high card points are a fairly crude tool for hand evaluation. Losing trick count can supplement HCP as an aid to hand evaluation, though it is much more useful when a good trump fit has been found than when responding to 1NT. However, I mention it as it helps validate your intuition that a fifth card in a suit is worth more than a third card in a suit, all else being equal.