It is not code compliant to install an oversized breaker for a 5 kW generation device rated at 5.8kW apparent, but oversized wire is a real possibility.
Many of our installations used #8 AWG all along.
Frack, Sunrun used #10 AWG on mine between the Powerwalls and the Gateway with the 30A breakers. I have three v2.1 with the 5kW (dunno where you find the apparent kW) nameplate.
I guess having 21 kW of juice is better than 15 kW, but I kind of hope I can just opt out of this firmware upgrade...
Edit: quite literally the only benefit I think I'll get with the 21 kW would be on my SGIP application. The large scale rebate goes up with 21 kW because then all the batteries fit under the 2 hour discharge tier. And there's 0% chance of me re-submitting for SGIP and dealing with this BS over a few hundred bucks.