Whimsical,
Waypoint had a motoring voltage of 146. You cannot use 50A per motor to convert to KW as 100A is the maximum current of both shafts combined. As I mentioned, current fluctuates and you cannot operate close to your ceiling without blowing a breaker. I expect the new 420's will have roughly twice the amperage and half the voltage.
As far as Waypoint's displacement, listed at about 8 tons (I don't believe this #), check out
Used Lagoon 410 catamaran for sale - Magic for a sister ship, or
Catamarans for Sale, Sailing Vacations and Catamaran services. For her performance in chop, I motored her into 6-8 foot seas with 25kts of wind on the bow making about 4 kts on GPS. Her power seemed satisfactory. Keep in mind that conventionally powered Lagoon 410's have, if I am reading correctly, the Yanmar 3YM30's, rated at 30hp. The "area under the torque curve" is much greater for electric motors, so a 12hp electric motor will likely outperform a 30hp diesel until it reaches higher RPM's. I don't know if the conversion factor should be .4 or .7, and I'm not sure anyone else has a rock solid answer on that, either. I recommend putting the theory aside for a moment and baseline your electric motor comparison on a vessel that works in practice. Lagoon 410S2's have both propulsion options.
Brett