Yes, the X6 mast has only 60% of carbon, but for the carbon extender they say it is not that important.
I'm afraid that in 10 knots it will be very difficult if not impossible to get on the plane for a 87kg rider with the IS107 + 7.8. For sure no fun and only downwind courses possible. It's even hard to pump my US147 + 8.8 on the plane. Normally I take my US147 + 10.9 in these conditions (8-12knots).
IS87 / IS101 / RRD114 / US147