[trit21]

I doubt that a single 16k btu unit will work very well at the helm in FL. I know of several people that have put AC unit at the helm (non Tiara) and they opted for a 24k BTU unit and it worked fairly well but still was not great on hot FL days, July August. 

In response to your idea of going from 12k to 16k in the cabin. If you do that, a 16k unit should have a larger supply duct than what is currently on your 12k unit. Not sure about the wiring, but that may also need to be upgraded. Remember that to feel 'cool' humidity is what needs to be removed. If you make the unit to large, the cabin will cool down but the humidity will remain and you will not 'feel' cool. Of utmost importance is to size it adequately to cool the space down will running long enough to remove humidity. I just replaced my original 12k unit with a new 12k Dometic unit. I was also thinking of upsizing but was warned about the very thing I just spoke about, humidity. Yes, the unit runs a little longer in the summer but it does a great job of removing humidity in the cabin for sleeping, so in the end I am glad I did not upgrade. 

Considering the original AC unit is on its own 30 amp circuit, I would say it would probably run both units but you will have to put sometype of interconnection box so they don't both start at the same time, it would not handle that I am sure. And, you will probably have to be selective on what is turned on and when, ie hot water heater, battery charger, stove, both AC units as I don't think it would everything at the same time. But, if you were selective on what is turned on from the AC power pane, I think it would work for you.