I believe 160/60 and 260/40 are the common plus sizes on the stock rims. I was looking at going this route with Metzeler Cruisetec. Avon Cobra Chrome is also highly rated but seems to be out of stock most places.
Don't have a 109 anymore, but on my Victory I have run every tire. Avon is my favorite, but based on customer and friend's reviews, my cruisetecs are sitting right next to me. Going to put them on Sunday. My wife has been running the Avons for quite a few years now and she told me flat out, "nope, not having it with the cruisetec, I want to stay Avon". We ride in the rain and they are great. If in fact the cruistec is better than the Avon, it is an amazing tire. LOL