A 16.5-inch treaty limit could carry G3 gun work into the Lion class

Would Britain develop better 16.5-inch guns for the Lion class if Nelson had used that calibre?

Yes; a 16.5-inch treaty limit would likely carry the G3 triple-turret design forward into the Lion class.

The historical 16-inch limit constrained the calibre available to later British battleships. With a 16.5-inch ceiling, the Lion class would probably adopt that larger weapon. Britain already had G3-era work on a triple turret, giving designers more than a paper starting point. Treaty wording could carry design knowledge from a cancelled capital-ship program into a later one.