Questions about new line

I have two readynas units, which until last evening were backups for each other. An older NV+ and a newer Ultra Plus 6.

Last night, the NV+ died (probably the notorious PSU, which I can't seem to find a replacement for), so I'm using the Ultra Plus 6, with data backup to an external USB-2 drive. I formatted the backup drive as ext-4 since in the past I've had trouble with using NTFS drives for backing up a Readynas, but this means if I ever need the backup, I'll have to do a bit of work.

So, I'm thinking about replacing the NV+ with a new system. I have to decide between one of the new readynas units, or a different brand.

A few questions:

Do the new units have any of the same expansion limits as the existing Ultra line (the 8TB/16TB limits)? The spec sheets suggest not, but then the published spec sheets and marketing blurbs for the Ultra Plus 6 didn't mention these limits either

Is expansion as versatile as some other NAS brands, which seem to let you expand with mixed disk sizes?

Can the new units backup to/from the Ultra Plus line pretty seamlessly using rsync?

Is Tivo archiving available? The spec sheets say it is, is this really true?

Finally, thoughts on the 316 versus 516? I think the 316 would meet my present and future needs, is a 516 worth the extra bucks?

Thanks!