Attachments are already cached on the "external" memory card.
Moving the app itself won't help much -- it doesn't have *that much* graphics (as games do), and "move to sd" mostly moves resources.
As for the database -- negative, for several technical reasons (I know K9 has this, but then it has a menu item to "recover corrupted database - last resort - dangerous - don't use unless you know what you're doing"). I'd rather not have that.
What I'm seeing on my (older) devices is that apps built into the firmware (Google's and third parties, like Facebook, SoundHound, etc.) get updates that have atrociously huge sizes, like 10-20 megabytes just for the app's code itself. Sad.
