In the GBA remake, the chest gives you a book called 'BUshido in the Bedroom', it's more of a joke chest then anything.

Atma weapon does damaged based on your current HP, I don't remember if you need low HP or High for it to do good damage though.

And I don't remember the Soul Sabre at all saddly.

And since you have the airship, I suggest getting Gau Back. Just take 3 members to the Veldt, and eventually Gau will show back up and join you as if nothing happened.