I added aftermarket Fly High X2 sacs to the rear compartments. They are 450 lbs each and about the max you'll get into the rear compartments. The dealer charged me about 400 bucks for install including parts and labor (I supplied the bags).
Timers are adjustable for fill and empty via the Murphy System, very easy to do. The dealer plumbed the system info the factory overflows for the rear bags.