I have tried to play my little solid mahogany Ohana with a group of Koaloa tenors and a Sceptre. I can vouch for them being very loud. On the other hand, I just recently started getting into bluegrass, and most bluegrass guitarists want a spruce top dreadnaught for loudness (mostly Martins). These are huge guitars with lots of sound, but I often read them saying on bluegrass forums that even a good Martin has a hard time cutting through a loud banjo. A ukulele is a very quiet instrument compared to most bluegrass instruments with steel strings and picks so stiff you can barely bend them.
Just based on what I know about the sound of koa vs spruce, I think before I decided a Koaloa Sceptre was louder than say a really good spruce top tenor or baritone, I would want to play them side by side. The relatively large lower bout undoubtedly helps a Sceptre be loud. I think that all other things being equal, both size and spruce make for loud instruments. It is said that you can drive spruce harder and it will still sound good, while if you try to play a cedar top guitar too hard it will get muddy. On the other hand, you can play cedar and probably koa at very low volumes and it still sounds good.
