No one can predict the weather on ANY stretch of water at ANY time of year. I have been across the Bay of Biscay in flat calm several times in November/December, but have had windows broken on ships in the Med during the summer months!
One thing IS for sure. The Queen Mary 2 is the best ship that you could ever hope to do the trip on! She has been built for that exact purpose, as an OCEAN LINER, rather than just a cruise ship! She can cruise along at around 30 knots with a specially reinforced hull and large computer controlled stabilisers. Just think before booking a balcony cabin though. With a 15 mph head wind, the airspeed past your cabin window could be around 50mph in mid Atlantic! Ever stuck your head out of a moving car or train window at that speed?