We went to Cape Horn in Feb/03 on Princess. From Falkland Is to the Cape is a 24 hour sail. During the night we were hit by a Force 9 gale. 35' seas and 50+mph winds. Very exciting. By the time we got to the Cape, wind and seas were minimal. We hit rough water 2 or 3 times - be sure and have sea sickness preventative. "The Patch" or scopolomene (check spelling) work very well with no side effects. Talk to your MD.
Our cruise only sailed the West end of the Straits of Megellan (in and out of Punta Arenas). It was windy in PA, but not particularly rough.