Hi dJOS,
I had a PW2 installed just over 2 years ago in Sydney from a reputable local solar company. My entire house is backed up including my central air (which is 12kW single phase). The circuit limitation I assume is more to do with the AGL offering (and fixed pay to installer) than anything PW2 related.
The backup gateway should come with the battery, as this is where the smarts are between the home system and the grid. No "sub panels" necessary during install. Like bogdan's install, my Solar can charge battery/run house during a power outage.
During the few power outages I have had in the last 2 I've never gone over the 7kW peak even with the ducted A/C going. I would only go over this draw with the AC + oven/cooktop on blast.
Also, if you get a Fronius or similar inverter that offers "pro" "premium" or other such paid subscription to track energy usage / weather predictions etc, I would say don't purchase these as the PW2 has everything you would want to track including a newly added "Energy Value" which is the amount you 'would have' been charged if you had drawn from the grid instead of the PW2. Also many integrations for better tracking/graphing.
Happy to answer anything about the setup/running if I can.
Nate