So, say the box remains sealed, upgrading from 1 10'' sub on 600 rms, to 1 15'' on 1200 rms, you should see ~6 dB increase? If you change it to a ported box then it will go even higher I'm sure. Just assuming considering double the power would be about a 3 dB increase and you are almost doubling the cone area as well, unless there is a law of diminishing returns, which I'm sure doesn't weigh in heavily at these lower power levels.
I always figured cone area and box design was one of the most important factors. Like, 2 15's seeing 300 watts apiece would be louder than 1 15 on 600 watts.