With regard to battery life, I often find that the batteries are being drained when OTG is connected even when I thought the unit was in standby mode. So my suggestion is to unplug OTG in those scenarios. I would try that and see where battery life stands.
The sdxc mod uses far less power than msata. It is also somewhat easier to get going. So if you can live with 256gb then sdxc is a logical choice. Above 256gb the msata comes out swinging. Even when 512gb sdxc becomes available it will be quite expensive
The X5 Gen II does have somewhat improved instrument articulation. And an improved amp design compared with the X3 Gen II. However, after a few minutes of grooving to the X3 Gen II you may not notice the difference.
I must conclude that my micro sdxc to sdxc adapter must have been the issue because I could not get the pny elite 256gb sdxc card to work using the extender in either the x3 or x3 gen two. I was formatting the card in fat32. Somebody mentioned using ntfs which I did not try.Anyway now I am mainly focused on OTG memory extensions not using the micro sdxc extender.