The larger ports will allow more gas flow, but the GX valves are smaller than the A15 valves, so you loose what you might have gained because of this.

The A15 head is very well suited to the A15 engine for almost any application except 'balls to the wall' race engines, however with suitable mods, they will serve well in this application too.

A modified A12GX head has more potential in a race engine, but again, really only in an all out engine.