But put it this way, would you rather have two slow ships that the older US ships can keep up with, or two faster more modern ships that would either be slowed by the rest of the fleet, or have to go out on their own.Certainly the Lexes had their critics, and you're right, the armor belt isn't very satisfactory. A high price to pay for that speed.
But once we're at this point...how much good does a pair of South Dakotas really do for the U.S. over the existing Standard battleships it already has? It's a rather incremental improvement.
If the US has a 50K-ish limit, more likely it scraps everything in the slipways, and goes back to the drawing board to design something to take full advantage of the limits - a true fast battleship, adequately armored - and yes, they'd have to think about 18" guns once they became aware of what the Brits had. Of course, doing that kinda trashes the whole Standard system anyway, doesn't it? Now it has two fast battleships that are much faster than the rest of the battle line. That will be useful one day for escorting carrier task forces, but they wouldn't be thinking about that in 1921-22.
The British don't have that problem as the have Hood,Saint Patrick(G3) renown and repulse all within the same speed.
Although R&R are significantly weaker.
Nelson and Rodney can be built with 15' guns,which means the can be faster