I've got the 6.5, 7.1 and 7.8. If you want the 6.5 for windsurfing as well as foiling you can't go wrong. I use them all with rdm, they have a great full shape and rotate faultlessly. They are also nice and light and have a long cut out for the boom for plenty of adjustment. The 6.5 is nice and stable even when overpowered when windsurfing but the 7.1 and 7.8 get a little out of shape and get backhanded in gusts on the fin. This would probably be resolved using an sdm. This isn't an issue when foiling though. Bottom end of the 6.5 might be as good as your 7 ncx. From past experience with the ncx they are great top end sails but not great in light winds but this may have changed in recent years.
For foiling only you may want to consider a foil specific sail.
Simmer quality is also a big plus.

I've now discarded my slalom/ freerace board's but still have a 106 FSW slapper for occasional use but it's not happy with 7mtr but would be good with 6.5 I think, so my 7 NCX is now only for foiling . My foil board is super light and easy to lift in very light breeze with my SS99 wing so thinking the 6.5 may give same early lift as 7 NCX but would be light in hand.