I'm not so sure if we (mortals) will feel the difference between 75% or 100% carbon masts. The best way to test it would be by rigging up 2 RSR (or RSS) sails, one with the X6 and the other with the X9 mast and actually try to feel the difference.
I think that the carbon percentage gets more important in the tallers masts (> 490)
This is just my opinion.
Probably I will get a lot of reactions now.
I'm convinced about feeling the difference in control between a RSR and a RSS sail. The difference was quite substantial.
PS: I use a 75% carbon Tribord mast in my RSR. It performs very well according to me.