RE: narrow boom?
I was wondering about your outhaul length for the 5.9 sail. Were you achieving the specification requirements of the sail, or is it your preference for some negative influence? Also, are you on the large or heavy side, as your harness line position tends to suggest you might be?
From the photo, it appears that you have a wave boom where the curve of the arms is reduced to permit the sailor to get closer to the rig to enhance maneuverability. Personally, I've never had an issue where the sail comes into contact with the boom, but I tend to rig my sail on the flatter side. If I'm correct in assuming that you prefer negative outhaul tension, you probably have two choices. Given the fact that most booms in the size range you're considering here will tend to be narrower than a somewhat larger slalom boom, the easiest solution would be to rig a larger sail to ensure that you have adequate power and enough positive outhaul to avoid contact with the boom. The other choice would be to purchase a wider slalom or freeride style boom instead.