I think 400x400 is too big for a single town (4 times as big), let's just look at a lot of towns: incomplete, unflattened, abandoned.... Additional towns exist for a reason and work just fine..just look at the difference between 200x200x16 and 400x400x16 (almost 2 million blocks difference).

In the other way, 5.6 million is just WAY too much on savings....Let's just take in mind that 1.12million Eco$ cost 500USD as a donation feature, so it would be like saving more than 2000USD + occupying a lot of space.

Most importantly, I think the point of expanding the map is not to fill it like lightning, but rather enjoy the new free space. (Even though it seems to be mostly water at this point)
We aren't really looking forward to a new expansion that will take 2 days to complete, are we? :p

 

Only advantage of this I can see if if your have a 400x400 town you only have to manage one set of perms
I know perms can be a pain but I rather smaller towns and such
If I could I would buy all 100x100 towns if they 20k or 200x200 for 80k
Same price just more perms to break up your town and prevent greifing