I used mine at lower power because I didn't use a speedlight, but a 320 w/s studio strobe...it uses a different flash bulb that is circular and rests inside the BD, unlike the directional end of a speedlight...A SB-900/580EX II, from what I have tested and understand, runs about 90 w/s, so it's going to be at a lot higher power.
Physically, the light output, in terms of evenness or spread, shouldn't change with the power output of the flash. You should be able to light the whole beauty dish with the flash no problem. I think, if you pull the flash away from the center, you're going to have less hitting the reflective area of the dish, not filling it as much.
The difference between the one you've ordered and the 22" Kacey dish is simply that you may be able get a little more smoothness because it's a bigger light source (less fall off, all else being equal), and a softer look with the white instead of the silver. I went silver because I felt like if I wanted a soft look, I'd use a sock. I eventually also bought a 22" dish and the look is different, but not significantly so, IMO. The white is a little softer/better for feminine shoots generally...