I think a more useful number (well, for me) is you need to double the power in order to get a 3dB increase in volume into the same load/speaker.
If you have a 87dB efficient speaker, it can play 87dB @ 1 watt (usually at 1 meter, which means less volume at your listening chair). 90dB at 2 watts, 93dB at 4 watts, 96dB at 8 watts, 99dB at 16 watts, and so on. You reach a ceiling pretty quickly, which is why, if you like it loud, speaker efficiency is every bit as important (maybe more so) as amplifier power.
The difference between a 100 and 200 watt amplifier, in total volume - all else being equal - is +3dB total volume. Of course, the more powerful the amp, the better able it is to handle transients at louder volumes. I've seen different numbers for transient peaks, with regards to required amplifier power - anywhere from 3x to 10x (i.e., if you're using 10 watts of continuous power at a given volume, you'll need anywhere from 30 - 100 watts in order to properly handle the peak transients in music/movies).
I'm going on memory, so hopefully this is correct. The gist of it is that you can never go wrong buying the most powerful amplifier you can afford.
You'll never use 400 Watts continuous in real world listening, but you may need a huge reserve to handle the peaks.
