I have seen a completely full TwinStore running two 1TB drives with just over 2300 albums on it. Meridian only suggest running up to 1TB drives as a maximum size so as to ensure utmost in the reliability from the system. However the TwinStores will accept and run 2TB drives, although these aren't available direct from Meridian.
So your solution for maximum reliability would be five TwinStores, each loaded with two 1TB drives, giving you storage for 11,500 CDs.
However 2TB drives are becoming increasingly reliable and I think you would be fine with 3 TwinStores loaded with 2TB drives giving you 13,800CD storage.
I would still recommend getting 6TB of raid storage to back up to, even though the TwinStores are Mirroring their drives!
We are just about to run our TwinStore up with 2TB drives over the next week or so. I'll report back once I have some news on operation and reliability etc.
The reason for the storage having to be in Sooloos land rather than attached storage is that it ensures speed, reliability and performance of the system as a whole as all components are 'known entities' to the Sooloos.
Ensembles weren't upgradable pre 2.1 firmware, but I understand that they are now, although I haven't any experience of adding additional storage to one myself. However unless you require the additional outputs that Ensembles would give you then TwinStores are cheaper, and have the drive mirroring, which the Ensemble doesn't.
Rick