I like the 750s in my 2012 A20. With that much weight in the rear the best thing for the wake was to add an on-bow sack in addition to the center tank and front bag. More weight in the rear just seems unnecessary to me. For reference, I don't really surf, just wakeboard.
The gravity drain does suck. I have the line connecting my bag and tank as short as possible. I've also tied the vent for the bag into the vent for the tank via a wye. I am also looking at replacing the rear drain pumps with dual inlet pumps made by shurflo.
https://www.walmart.com/ip/Shurflo-Piranha-Livewell-Pump-12VDC/33276573. This would pump water from the tank and bag simultaneously. Not sure if it will drastically improve time, but at least it would be one push of the button and then done, instead of the constant on/off that we currently do waiting for the bag to drain.
For this set up, it would go fill via the top of the bag and then drain through the drain pump into the tank. No need to turn the drain pump on. The tank fitting currently connected to the bag would have to be capped.