I think it depends on the boom.
I have a Maui Sails 190-240 carbon boom and I would not hesitate to use it at 238. At 218 it just does not move. As a comparison, my other older carbon boom is a 180-226 and felt a bit soft when extended at 204 with my old 6.9 and even at 194 with my old 6.6. Maybe it would hold perfectly well full extended, but I would not feel safe to try.