I have the NV+ as well, and as far as I know, "X-RAID" is a feature in all of the ReadyNAS products.

It is a traditional RAID setup still, so rules about identical drive sizes apply. Though X-RAID2 in the newer units will open up the new space once enough disks in the array have been upgraded.

Digging deeper into how to recover from disaster on a ReadyNAS, I did find it would be possible to plug all the drives into a normal Linux machine and get the data back. So even if the device does implode and a replacement can't be found, I should be able to get my data back. I was initially concerned about this, as the NV+ has a hardware raid chip in it to keep speed up, and those can sometimes be very proprietary and closed off.