I'll try to answer your questions by telling you my own experiences. I have owned three koa guitars, including the one I currently own:
1. Santa Cruz H - all koa. This was a lovely guitar, especially in appearance. However, the koa top, combined with its small size, made it too quiet a guitar for me. After several years of ownership, I sold it.
2. Larrivee OM - sitka spruce/koa. This was much more fulfilling in the sound department, due to the larger body and spruce top. I would still own this if not for...
3. Taylor GA - sitka spruce/koa. This 1995 beauty , in near-mint condition, was sold to me from an estate sale, and I sold the Larrivee.
In summary, the larger the body, the better koa seems to project. The spruce top also provides better projection than a hardwood top like koa, certainly when new. Last note: the older the guitar, the more responsive it becomes. This is true with most guitars, but I have found it especially true of koa. New koa guitars sound somewhat midrangey, muted and tight to my ear, but as they age, they really blossom, and take on a more mahogany-like tone. There's still lots of midrange, but with more sparkle and bass thump (similar to rosewood or ovangkol). So while koa is a visually pretty wood, you need patience in terms of the opening-up process. Koa guitars can sound good when new, but not as loud, strident or resonant as some of the other traditional tonewoods. Based on my experience with the guitars above, along with the many I've played along the way, It's nice to have a koa guitar in the arsenal.
P.S. not meant to be a plug for Taylor, but newer Taylor koa models have sounded pretty impressive to me right out of the box. I think that Taylor has finally figured out koa in terms of desired thickness and bracing to bring out the best that the wood has to offer, even when topped with koa. Hope this helps....