RE: narrow boom?
The short HPL carbon booms are among the narrowest around. That said, if you don't use a particularly deep drafted sail, you should not run into problems with the sail touching the boom. Ive run lost of different wve and freestyle sails from 3.5 to 6.3 on my HPL without the sail toucing the boom, but my 6.6 speed sail (which like s a fair bit of neg outhaul) and my 7.0 freerider/wave sailo doers touch the boom.
As SteveC says, the reason for the narrow width is better handling in manouvers. If you're more into freeride and straighl line blasting, a slightly wider boom might work better, but even so, I'm also surprised a sail as small as 5.9 will touch the boom. It makes me suspect the sail is either under outhauled or that the boom is longer than neccesary for the actual outhaul applied.