I need to optimize a page covering the topic: "Giorgio Armani's suits"
The best possible title would then be "Giorgio Armani's suits".
Remember, even if your entire site is about designer clothing you can only rank one page for that term (with the possibility of a second indented page). Therefore you don't really need the words "Designer Clothing" to be site wide. The term is indeed extremely hard to rank top 3 for so placing those words on your index page (which is most often your most powerful page in serps) makes sense. I'd leave it off the other pages.
You might want to test out the Google keyword tool too, I would think that "Armani suit" or "Armani suits" would be more searched for than "Giorgio Armani's suits". You might also want to look into copyright issues if you really want to place that name in the title of your index page, he may not like it :-)