Does wattage means better "coverage"? ...
In my opinion, no. What a higher wattage does mean is that you can mount the fixture higher, and get it to cover somewhat more area. Higher wattage also means more light directly under the fixture, so you can keep high light corals.
As for the Kessil AP700, manufacturers tend to be very "optimistic" when it comes to the specs of the lighting. I think putting it on a 48" long tank would be asking a lot of the fixture, and that you'd tend to get dark corners. I should point out that many people might be very happy with this. That would bother me, and I'd tend to want three of their more conventional fixtures.
If you look at the spread of light from an LED fixture, it tends to beam straight down. This tends to limit the length they will span. Of course fixturers that have multiple clusters of LED can span a bit more,
