It really depends on what you'd like to keep in it. If doing sps for example they would only be ok directly under the light very high it the tank. par values around the edges of your tank would be super low, almost nothing. Ai has a graph on their web page showing this. I'd check it out and then check out your corals lighting needs!
Yeah, it would be iffy for "full" coverage. I always recommend 1 prime per 12" of tank length. I have 5 of them on my 60". I have no problems growing various SPS etc.
12" sounds about right. I tried a single prime on a 20" tank and most sps frags had to be moved high center. I ended up suplimenting with t5's down the road to get better coverage