It takes 2x the watts to get 3 more dB of sound. If you want 6 more dB, it takes 4x the power, or 8x the power for 9 more dB.
So if a speaker is 3dB more efficient...
87dB sensitivity --> 1w = 87dB, 2w = 90dB, 4w = 93dB, 8w = 96dB, 16w = 99dB, 32w = 102dB, 64w = 105dB, 128w = 108dB
90dB sensitivity --> 1w = 90dB, 2w = 93dB, 4w = 96dB, 8w = 99dB, 16w = 102dB, 32w = 105dB, 64w = 108dB, 128w = 111dB
That's just one speaker, but in stereo you have two speakers. The sensitivity are rated at 1w/1m, so doubling the distance to 6 feet or so is basically evened out by having 2 speakers. Of course you'll have room interactions, but that works evenly with all speakers.