I personally don't know anything about the "xso" chains, I run o-rings and am happy with them, as far as the vortec aluminum sprockets go, i dont like them because the teeth wear down alot faster than steel sprockets, that and as I've stated before I don't like the 520 conversion due to the fact that a smaller chain = a weaker chain, ive had a 520 break on my 600 so i can imagine on a 12, as far as the gears on a 1200 im kinda in the dark also but just from experience with bikes, taking your rear sprocket down to a 44 and leaving the front at a 15...all its gonna do is keep your top end at a higher speed, but how often do you go top end?Some of the guys will prob help you out on exact gear ratios but a 47 or 48t rear and a 14t front should be what your looking for...and by the way, your local bike shop should be able to get you out the door with a new chain and f & R sprockets for about $165 if not better, go with what you know, and if you dont know at least you can take it back to the shop you bought it from....
And to the other guy, if you have a gen2 bandit, you should have came stock with a 14t or 15t front but your rear WAS a 47....