Hi waiting for wind, I too use the X6 mast. They are sweet masts without that fear of breaking the ultra light masts have. I use the 490 X6 in the 8.4 ( it's 90% carbon anyway )and the 460 like Carl.I think that is 80%. As you get smaller I believe it matters less and less as it comes down to control.
Having riden the 8.4 RS Racing my gybes have improved especially the alpha speeds. These sails are a lot easier to rig than the RS6. It is not to hard to over downhaul. If you feel your back hand is pulling in to much your probably over downhauled. No need for winches just do it by hand. Up to now I've been using the X6 boom as well, but just got an X9 100% carbon. On the shore it's difference is quiet noticiable. I'll put up a report on the boom after a couple of uses.
You'll love the feel of the Rs Racing sail, The speed on all reaches and gybes are second to none.

