More efficient and just as secure to sign message hash using Ed25519?
Peter Schwabe, one of the authors of Ed25519, directed me to a recent paper titled "EdDSA for more curves".  The section "Security notes on prehashing", page 5, says that the Ed25519 algorithm without prehashing the message is resistant to collisions in the hash function, while using the algorithm with prehashing is not.  Of course the hash function is not supposed to have collisions, but if it does, not prehashing gives added protection.