Couple of interesting points. First of all, my 2kWp system (brand new, Netherlands) at €1300 investment pays back for itself in like... 4 years? We have one-to-one payback. With second hand (unused but older) German panels you can get down to 2.5y payback period. That is batshit crazy; it means that even when lightning strikes and completely destroys your uninsured PV system after 10 years, you'd still have had a couple hundred percent ROI. With the average system lifetime being about 19 years around here (and rising, because newer systems are generally much more reliable than the early-2000s systems), there is zero financial reason not to install solar panels. Better still, the government has promised not to decrease FIT in the next 3 years. You'd either have to have objections against the aesthetics or an extremely unfavourable roof orientation/shadow casting to not install solar panels here.
Of course, things are quite different abroad. I'm amazed at your 7.5y payback period even at relatively high installation cost and low FIT. This probably has to do with considerably more sunshine (2500ish in Sydney vs 1600 for the Netherlands).
Battery systems have fallen out of favor really rapidly because of the freefall pricing situation that solar modules are in. A sufficiently large battery, even the cheapest flooded cell lead acid batteries (~$0.10-0.12/Wh), will cost about the same as the solar installation, have quite a low efficiency (80% typical cycle efficiency at very low charge/discharge rates, worse with a smaller battery) and basically at this point will never pay back for itself. Even with these big differences between FIT and consumption cost. It seems fine at first glance, but a lot of people don't grasp the engineering behind battery systems. For instance, you can't efficiently charge lead acid batteries at high rates; they only achieve reasonable efficiency at C/30 to C/100. So you should install a battery that is 30 to 100 times as large as your actual energy need. At 20kWh/day (for a 3-3.5kWp system in AUS), that's a 600 kWh battery or about $60 000 - without chargers, without inverters. And even then you will still lose a couple tens of percents in efficiency losses. Lithium ion chemistries fare a lot better and can be scaled to C/2(ish), but they are about 5-10x as expensive per kWh and have slightly less charging cycles (about 2500-5000 at such conditions vs 10 000 for PbZn). Either way, lots more expensive than the actual solar installation, so your payback time on something like that will be tens of years or a century or something.
There are ways to optimize a battery system, undersize it and all that jazz but it'll never be cost effective unless energy prices are really high.