We went around the Horn in late Feb. as part of our 16 day South America Antarctica trip. We were very lucky, and coming from Antarctica, we had the "Drake Lake" effect, barely a ripple, and likewise for rounding the Horn. The Star Princess did two such trips this year, January and our February one. On both trips the Horn was smooth going. I would take a chance, as it is a wonderful trip. I do not know if your itinerary includes Antarctica, but ours included four days of Antarctic cruising, and that was the experience of a lifetime. I would recommend you consider an itinerary that includes this. However, even if you do not do Antarctica, you will love the awesome, breaktaking fjords of Chile, and you will have wonderful opportunities to see penguins at different ports.
If you are prone to seasickness, I recommend investing in an electronic relief band (about $130 or $140). If you cruise a lot, you will get your moneys worth. I swear by mine.
My advice: Take the chance and do it!