If both you and the iMessage counterparty both have e2ee enabled for iCloud Backup, it's unlikely at present that the feds can read the messages without a warrant under FAA702 "foreign" intelligence collection (which as we now know thanks to Snowden is routinely used against even Americans).
With an actual warrant or wiretap order, there is no guarantee that Apple can't or won't insert additional endpoint keys into the conversation.
But, that said, 99.9%+ of users don't have e2ee enabled for iCloud Backup, so the e2ee in iMessage is mostly irrelevant, and it's available in full to the feds at any time, no warrant required.
tl;dr: use Signal.