Doug,
I'm gonna do the same thing to my new SX when it arrives from HyFlyin. The big problem with the SX ballast system is the small size of the solenoid valves that they used, they are 1/2" valves. Bigger valves are prohibitively expensive. The valves are necessary if you don't want to have to turn a hand valve after filling each time. I don't like to drive around relying on vinyl bags and hose to keep my boat from filling up with the lake.
You might think about copying the Supra system. They use a PVC manifold fed by a LARGE aerator. They use a Rule 2000, I would probably use the rule 3800. Then, 1" sprinkler valves are used off the manifold to feed each bag/tank. Aerators on the outlets. You save money and wiring having fewer pumps, switches, etc. The Rival system uses a 12vdc 1.5" RV dump valve on the inlet of the manifold. You can get one of those for 100$. I don't think I would pay the extra 300$ or so that you would pay for one of those packaged systems. You can build the system yourself for a bit less money.
The Supra method is simpler, probably not as fast to fill, but still an improvement over stock. It is also easy to expand if you want to.