This latest leak will also go by firmware version is I9100XXKI3, which has been translated to European version 3 which was dated September 15th. Obviously, if you think that you have the guts to go for this leaked ROM, make sure you perform a backup of all your important data as you can never tell when it might just go all wonky during and after the installation process.
After all, just because something has been proved to work does not mean it is 100% foolproof – you would do better to wait for the actual update from official sources. Anyways, if you are patient enough, then you can see Android 2.3.5 Gingerbread arrive for the Galaxy S2 in due time via an Over-The-Air (OTA) update.