Thanks for taking interest in my question.
For the luff leght I think It would work, and let me downhaul correctly. In fact I bought the sail in 2003 right because I had a North Sting 490/28 (30%) and this carbon extension left by the short life of my Neil Pryde Ultralight 7,8.
So with a stiffer mast than 490/28 I think there would be no problem from mast+base extension.
But I don't know how tip extensions work, base extension usually affects a little the bending curve, but tip section is on the most critical section of the mast...
About crushing the mast if you could see this FS you coud agree with me that is one of the strongest mast ever seen, it seems way thicker than other Fiberspar than I got, and in fact the top section doesn't slip without problem inside the base as with all my other FS sdm masts.
Sail is HSM Stealth and new North Masts are not recommended for HSM, different bending curve. And this FS 5000 90% was bought perfect for 86 euro...