A Japanese 510 mm gun was technically workable

Would testing a 510 mm gun have shown Japan that it was impractical?

No; it was technically feasible in large twin mountings, though it imposed a slower rate of fire and major design tradeoffs.

A roughly 5,776-pound shell did not make a 20-inch gun automatically impossible. Japan's move to roomy twin mountings kept the loading problem within contemporary engineering capability. The rate of fire would be slower, though still measured in rounds per minute. Its practical value depended on whether the desired range, penetration, and explosive effect justified the mounting weight and other ship-level penalties.