How to free up Message's "dead space" while preserving some conversations?
There may be a "bug" in IOS that hangs onto old messages or attachments even after a message or thread is deleted. The discussions you link to on macrumors.com seems to indicate that.

That said, if I have a lot of space being taken up for no apparent reason I find the best way to fix it is as follows.


Back up your phone, Apps and all to your computer (encrypt the backup, it makes restoring easier)
On your iPhone fo to Settings > general > reset and tap on erase all contents and settings
Once the phone has been wiped, restore it using the backup you just made.


No guarantees that this will do the trick but it has successfully worked for other apps for me in the past. It would be interesting to know if it fixed the issue for you as well.