It can be turned into 2/3, or something around 1/2 in practice, if signatures would be weak, but if they would also carry some data. For example: 128-bit signatures are weak. You can have the first half of the signature, which would just make it weak, and inform about the order, and the second half can carry some data. Then, by using lattice attacks, private keys can be easily recovered, so you can read everything.

If a given elliptic curve would be fully broken, you would reach 2/3 spamming efficiency, where for every 96 bytes of consensus data, 64 bytes would be taken by the payload, and 32 bytes would just make things tick under ECDSA. But in practice, under lattice attacks, I expect the spamming factor to be closer to something like 1/2, because you have to leak some bits somehow, to make going from public to private key easy enough, so your data could be read by anyone.

And then, spamming efficiency of 2/3 is just the same as in Bitcoin with P2PK, and nothing else. But 1/2 spamming efficiency, or even 1/3, is something practical. I guess some spammers would happily waste 3 MB of MW blockchain space, just to publish 1 MB of data in it.