I was aboard QM2 on a cruise from New York to the Carribean in 2008, and we made 29+ knots in rough seas for over a day, much faster than she does on transats, and she handled it magnificantly. This was likely with Diesels and Gas turbines at close to full boil, seeing her quad screw wake at the speed was extrordinary. Cunard have since slowed her down to reduce costs, but QM2 is every bit a liner unlike, anything else in today's cruise fleet.
I was aboard for her last 2011 crossing this past December in the three days of the roughest seas I have encountered in years, and she again handled it magnificantly. I have some incredible video shot from the library and Commodore club of those seas.
World economic conditions, including oil prices, were dramatically different back when QM2 was built, but they built her and she is all we have in terms of a true liner. I'm going to continue to sail on her.
+1. Let's be glad we have QM2, whatever the trade-offs and compromises.