The initial post about the Autumn+ version can be found
here (click) and you can find the mod
here (click).
Once in the folder, choose your device (ZX300 or ZX300A) and then choose the variant coresponding to your device. Each ZX300/A has 4 variants: China, Japan, Europe or Universal. If you're not sure which one is yours, you can just download all four of them and try them. The ones not matching your device will just not detect the device, and they will not let you continue with the update.
Please note that you should have the 3.02 stock firmware already installed.
To summarize, this is just like installing a stock firmware from Sony.
This consists only of tuning modding. The stock firmware stays the same.
Yes. Just install the stock firmware and that's it.
There shouldn't be any, but this is only my opinion. Compared to other mods (for other devices at least) this one is pretty balanced aka it doesn't seem to put any significant strain on the device. If this happen, you usually hear the sound being a bit too loud somehow, forced. Because of this, I assume that the battery capacity shouldn't be affected in a noticeable way.
Give it a try and you'll notice how smooth it is. This is the reason why I don't think it's putting any significant strain compared to the stock version.