Answering my own question...
It's hard to find details on EI's configuration for the 757, but there isn't too much variation in what is offered in a 2 class configuration, so it should be around a 20% boost in seat capacity on their Transatlantic routes out of Shannon.
You're right that there's not too much variation - it's typically just a question of how many rows of business class.
This suggests the configuration will be C12Y165.