I doubt it would give you better performance. The carb bores are the same size. The jetting is still not optimum (it's not optimum for the A12 power, either). Be aware that some newer A12 carbs have smaller bores.

It should run OK on an A15, but the A15 air jets may be different, and so the A15 carb might actually run richer at full throttle. Who knows? Dyno your car before and after, and tell us how it goes