I had a deluxe twinn and I have a few opinions on some of your upgrades. I liked it, my girlfriend loved it, because we could cycle around and she didn't have to navigate through traffic on her own. She usually looked at real estate as I 'biked' up front. We cruised to cafes, the park, the beach, and so on.
It isn't really a performance bike. Alloy rims seem like a waste of time. I don't think an SA-3 is hearty enough to handle near double the torque, you rarely see them on tandems. I don't know about the SA-drum. I do know about the atom drum brake, it's Fred Flintstone brakes at best, but did the job for our combined 350 lbs. The magic of the drum isn't stopping power as it is its durability.
With what you have I would true and tension the rims, replace the stokers crank, button it up the best you can and try it out with your significant other and see how you and she likes it. Then You can access what you have, how it works and what you may want to upgrade.