Wheel spacers=extra crap to go wrong, and it puts a lot of undue stress on your outer bearings. Besides, for that kind loot, you can buy a set of aero black steelies from summit, and have them in 4 or 5 days. Also, 15 or 16" doesn't matter, all you should be concerned with is the outer circumfrence of the tire. Unless there is something i am missing on the disc brake set-up on the Rubis, it shouldn't matter, and you would get more bang with 15s.
I linked all the stuff to you in a PM, but basically get a 3-3.5" backset on your trail rims, and get a decent set of LTB swampers, and keep your nice MTRs for the street, and you'll be cash ahead. You're only going up another 1.5" so the 31s you're running now won't look too bad, and you'll get better MPG with the smaller ones. Your speedo is calibrated to correctly read with your current tires, also.