What a great adventure. We were on Royal Princess in Feb/03. From BA to Falklands it was quite calm. From the Falklands to the Cape Horn (24 hour sail) was quite a ride. We hit a Force 9 gale (50+mph winds and 30 ft seas). Ship was very seaworthy, but did rock & roll. Once at the Cape the wind calmed down to 10-15, but it was cold only about 4 or 5F. Also, we had a bit of rough water on the Chilean coast, but nothing too bad.
If you are concerned about motion sickness, my wife found the magic cure. She is very prone to it. Had we not found an antidote, we could not even have contemplated the trip. We found a phamacist who could take the active ingrediant in "the patch" - scopalemine (check spelling) and put it in a cream form. There can be a problem if you have a sensitivity to the patch adhesive (like a band-aid). The cream came in a plastic syringe (no needle) and you just dispensed a small amount on your finger (where a surgical glove - cut off the fingers so you can use one pair 10 times) and apply behing your knee. The skin is thinner there - quicker absorbtion. Don't get it in your eyes - it dilates the pupils for 12 hours! There are no side effects - use every 8 - 10 hours.
Bad weather is part of the romance of the sailing to the Cape. In a very small way, it gives a feeling for what those old mariners faced. Imagine climbing a mast in 30 foot seas, reefing a sail in 50mph wind while wearing canvas clothing, no shoes or gloves. So some people missed a meal or two. I wouldn't have exchanged that trip for anything - a unanimous feeling amongst the cruisers.
Good Luck - contact me if want any information regarding ports, tours, etc on this trip.