Just to point out, this will not increase the damage of your nukes at maximum power; rather, it increases the accuracy of your nukes, and does much more damage that way. As a RDM, my nukes fail more often than I'd like, and this thing makes me about as nuke-able as a BLM (just with less spells). Unless you're going for Fenrir, and you're a mage, give this a shot. :D
Oh, and don't sweat the MP reduction. This takes some skill at equipment swapping, but first do your regular spell routine without the ring until you've reduced about 15% of your MP, then stick it on. Remove before Converting. No MP loss whatsoever. :)