My .222 Rem with 14" twist shows marked group widening moving from a 50 grain flat base Hornady SP to a 52 grain Sierra MatchKing boattail. The reason is the difference in bullet length. Length matters much more to stability than weight does. Indeed, if you can hold length and velocity constant, increasing weight improves stability. It is only the fact increasing weight is generally associated with increasing length, and to a much lesser extent, with decreasing velocity, that it causes folks to think you need more twist for greater bullet weight. Weight is not the reason. If you melted out the lead core from a bullet jacket and poured if full of gold, which is more dense than lead, you'd find you needed much less twist at any given velocity, not more.
If the manufacturer says his bullet won't stabilize in a 14" twist, then you can probably believe they've tried it or had unhappy feedback from customer's who did. They'd have no reason to discourage you from buying their bullet otherwise. It is also a fact that many rifles don't have very accurate twist pitch. It would not be terribly unusual to find a 14" twist that was actually closer to 13.5" or to 14.5". This accounts for why some guns shoot bullets well that others with the same nominal twist do not. So the exact bullet size that your gun can deal with is going to be somewhat individual.