All else equal, my guess is that it will sound a bit brighter and better than all koa.
But regardless of the back/sides, whether a koa top will sound good or not depends primarily on the builder. With softwoods, you can just follow a recipe and it will usually sound pretty darn good. Hardwoods are not so forgiving.
Larger size also raises the difficulty, so definitely go with the GC.
I would also go with a lower density bridge like Indian or Brazilian rosewood, and shape it to minimize mass. But I don't do scalloped bracing, and a heavy bridge may be part of what makes that work, so trust your builder on the bridge choice.
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