16tb Limit - Is that Useable or Total?

OK. My plan is to use 2 x 1.5tb drives I already have and put in 3 x 3tb drives that I will buy new.
So this should give me 6tb double redundancy (real about 5.5tb).

So if I then replace the 2 x 1.5 with 3 x 3tb = 6x3tb I can go to 12tb (11.1tb).

But I can't then swap them out for 4tb drives (when they arrive), as that would be 16tb (14.90tb), which is more than (5.5+8 = 13.5tb), even though it's below the 16tb max.

This 8tb max upgrade problem seems to be a problem with the EXT4 filesystem

http://www.readynas.com/forum/viewtopic.php?f=51&t=51475&start=15#p294602 Most of you are probably hitting the current limitation of EXT4 64-bit filesystems (which we use on all of our x86_64 machines) cannot do online expansion by any more than 8TB over the life of the filesystem.

It looks like this can be overcome by doing it offline.
http://www.readynas.com/forum/viewtopic.php?f=51&t=51475#p294588

Is this true? I don't have anywhere close to 16tb of data, so this isn't a real issue for me yet, but I did have a 20Mb hard drive on my 286, and that seemed like a lot :D

The other option is to just get an extra 3tb hard drive at the beginning, giving me 9tb (8.3tb) which I can up to 16tb.