Going through old bridge books (by e.g., George Coffin), I was struck by the fact that some 3NT contracts made easily with "only" 24-25 and a good five card suit, while if both declarer and dummy had flat (4-4-3-2 or 4-3-3-3) distributions, a 3NT contract could be difficult even with 28-29. Intuitively, it seems logical that a fifth card in a good five card suit could be worth a king or even and ace, that is, the difference between 24-25 and 28-29. It almost seems like the "standard" 26-27 high card point (hcp) requirement is an averaging of these two polar cases.
Of course, there are five card suits of varying caliber. All other things being equal, a 5-3 fit is much better than a 5-2 fit (unless you have three or four of the top honors with the 5-2 fit). And a "5-3" fit of xxxxx opposite xxx isn't worth much, except as a stopper. But here, we're talking about a normal eight card suit situation with more than half the hcps between declarer and dummy. That is, six or seven hcps in the suit.
Has the advantage of a five card suit been demonstrated either empirically or by experts?
Empirically, are there databases of e.g. tournaments that show that more than 50 percent of 3NT contracts are made with, say, 24 hcps and good five card suits (or even random five card suits), while you need many more hcps to have a better than a 50-50 chance of succeeding if your longest suit is four cards in one hand?
For expert views, are there any experts who say, if you are responding to 1NT (15-17) with a hand that has nine hcps, and one five card suit of say, KQJxx feel free to jump to 3NT which partner is likely to make even with a minimum? Whereas you should hold back to 2NT even with ten hcp if your best suit is only KQJx? Alternatively, would such an expert advocating raising a response of 2NT to 3NT with say, 16 hcp and a suit like KQJxx, while passing 2NT with 17 hcp and KQJx as your best suit? (In these examples, your main suit has the same honors in each case, and differ only in length, by one "2.")