Chuck, I've seen a number of theories on sizing AC units for sailboats. When I talked to the Webasto guy at Annapolis last year he thought the 9K might be a little undersized on really hot days and recommended the 12K.
I agree that a 9k is going to be a little small on a hot day. But for me, I'm more concerned about my sleeping comfort and at night, an oversized unit will cool down quickly and the compressor won't run much during the night. When that happens, your humidity levels will go up. When it comes to comfort, low humidity is just as important as low temps.
I know because that is what is happening to me now. I put a 16K in my Catalina 34. I wanted the 14k but it had been discontinued so we went to the 16. I have to keep my thermostat in the low 60's to keep the humidity levels down.
Some years ago, we had a Catalina 30 and we used a CruiseAire 5k BTU unit in the hatch. Didn't do much during the heat of the day, but once the sun went down, it got comfortable and if I had turned the thermostat all the way down, I probably could have had the temps in the mid 60's by the next morning.
Based on our experience with the 30, I think the 9k unit would have worked great.