Cutting the top 50cm off of my Neil Pryde X9 580
to make it a 530 which is the length required for my
Neil Pryde RS5.
Or just cut off 30cm making it a 550 as the RS5 has
a luff of 551.
Or is it a stupid thing to do?
I think you'd regret it. Any more than about 20cm moves the mast enough that the boom may no longer be in the correct reinforced area and you break the mast.
Full carbon race masts break enough without encouraging them 
So 20cm hmm yeah if you want to, but you now have zero resale value.
30cm maybe if desperate but could move the boom enough
50cm definitely no way
I'm going to go against everyone else just for the sake of it.
The only caveat I would have is if the mast curve is similar enough.
If you want to try and keep the boom area in the same place, I think cutting it from the top would work. Those sails are so big, and generally the boom cutout is the same on a 12.5m as a 10.7m. If you were really worried about it, you could cut a bit from the bottom and a bigger bit from the top, but I think if you laid the sails out, the boom cutouts would be in the same area.
I have broken a 580 and a 520 (or 530) on the beach, so I concur that just looking at these things can break them... but if you have no use for the 580, there's not much of a market for them.
Just for the sake of it, I found my 530 and 580, and measured them. The 530 is 20cm shorter on the base and 35cm shorter on the tip. Don't ask me why those numbers don't add up! Surprisingly after sitting in their bags for 5 years they aren't broken which must be unusual for a NP formula mast 
