Is optimistic data allowed on onion services? And should it be?

While working on Arti, I've noticed that optimistic data on onion services might or might not be officially allowed. On the one hand, it will improve performance, since the client won't have to wait for a CONNECTED in order to send data. On the other hand, it smells like a possible side channel vector (q.v. proposal 344) if the client chooses a BEGIN message that they know will provoke an END but not cause the circuit to close.

cc @mikeperry: what do you think?