I'm re-doing my 180 shortly and am going through the same options you are I suspect.
I ordered a single 360W (and an Apex controller - probably should stop drinking beer and ordering stuff late at night) last night, so that I could assess coverage and color. I've either got a chunk of my main lighting or the world's most expensive refugium light on order - time will tell.
My current thinking, assuming that I like the Kessil, is to do one of the following:
1) Go with 3, covering 2x2 each.
2) Go with 3, covering 2x2 each, and supplement with some of the many T5's I have lying around.
3) Go with 4, spaced evenly.
4) Go with 3 or 4 on a light rail (which I also have lying around).
I have had reasonable success with my current T5 setup, but truth be told I have always missed the shimmer that I had with my smaller halide setups.
Once I see the first one in action I shall know more.
Dave