I just bought a new GS Mini Koa and was thinking about putting light strings on it as well when I saw this thread. So great timing.
I very much admire the knowledge of the folks on this forum and have made many decisions based on their feedback.
Since I am in the unique situation of owning two GS mini koas-- more on that later, I guess

-- I thought I would experiment with this directly. So I put some Martin Acoustic Sp Custom Lights (.011-.052) on my older mini. I have fell in love with those strings and they are on all of my full-sized guitars. On my new mini are the factory Elixir medium strings. I recorded a video of me playing the exact thing on both guitars to compare the difference. First I did fingerstyle, then I strummed.
Surprisingly, I found no volume differences between the strings. As a matter of fact, the lights actually sounded a bit louder while strumming, which surprised me very much. I still prefer the sound of the lights so I will likely put those on my new mini when I upgrade it with Gotoh 510 mini tuners and replace the bridge pins with abalone ebony pins. The tuners arrive next week.
I don't claim that this was a perfectly conducted scientific experiment (that's what I do in my real job), but I found the comparisons interesting.
I hope this helps.