Suppose the sea were 15 feet lower?

Tsunami is Japanese for "harbor wave". They cause no damage at sea but are devastating when they crest in shallower water. With the sea 15 feet lower, the wave would have crested earlier. and that might have reduced damage from the wave, although not from the earthquake.

BTW, isn't 15' a bit extreme? At least for the foreseeable future?

Yes the damage would not have been further inland. But yet it goes both ways so assuming that the sea was 15 feet lower we would assume it has always been like that, so places in Japan which are currently and always have been underwater in your hypothetical scenario would be now on the surface an potentially people and buildings could be there. So yes what is damaged now wouldn't have been as damaged but possibly other areas could have been damaged instead.

Well given that humanity has tend to build closer to the ocean, if the ocean were 15 feet lower, I am sure we would have moved there and there would be as much death. As for the 15 feet claim, we are currently seeing a rise of 3mm/yr, to get to the 15 feet that you are talking about would thus take about 1600 years. While this frightens me to no end, I would not cancel any future vacation plans based upon the scare-mongering of the alarmists.

not much less, the amount of energy produced by the quake will be the same no matter the amount of water, so the tsunami will be lower in height but stronger.