Hi sea- shell,
the only thing i can say, is that i've experienced the Opera as an extremely smooth moving vessel. But we have never faced rough seas during our cruise.
It is hard to predict weather- and sea- conditions on a transatlantic crossing, so it is a bit of a gamble...at least i can say that i liked the suites (which are more or less "Junior- Suites") especially because of their location on the top deck forward.
If the seas get rough, she offers plenty of comfortable seating midships on lower decks, behind the atrium. But consider, she is by today's standards a relatively small ship for a TA, so there might be a lack of entertainment on those many days at sea.