Danno,
Just because a mast is sealed, doesn't leak, doesn't mean the boat won't turn turtle. There are production catamarans
in the publics hands that turn turtle with sealed masts because the mast is too small in section; not enough cross section area per unit length in the mast to stop the boat from turtling. In your turnover tests, have a few empty and sealed gallon jugs available to tie onto the top of the mast and add jugs as necessary until the boat does not turtle. Then you know what it takes to stop the boat from turtling. I hope you are lucky and it does not turtle at all.
Here's the easiest way to right a turned over catamaran laying on its side. Throw a drough chute off the bow tied to the bow to hold the bow into the wind. Sheet in the mainsail. Push the main traveller car down to the water. The mainsail will inflate as the leech approaches the water. The lift generated by the mainsail will right the boat. The drough chute off the bow will hold the bow into the wind. When you are ready to continue sailing, retrieve the drough chute and store it in a tramp pocket.
Good Sailing,
Bill