I don't think I've seen that combination; sounds interesting. The neck woods contribute the most to the sound, then the body wood, then the top wood. I'm just guessing here but I'm thinking that the Walnut neck laminations will be less bright than an all Maple neck; but that the Birch body may add some brightness over a Mahogany body. And I'm guessing the Koa's sound would be similar to but maybe a little brighter than that of a Walnut top, but less bright than a Maple top. Sorry, maybe if some of the other guys see your post they can add better descriptions.