Ken, I'm doing the exact same job today. Since the
backsplash is not subject to tremendous amounts of moisture, as in a
shower, it will do fine to put directly over drywall. Keep in mind, though, thatwhen it's time to replace it, you will be putting in new drywall. With
backerboard there's a possibility you'll ba able to remove the tiles, unscrew the backerboard, replace it with new and retile. The other possibility is, of course, it may not work that smoothly and you'll end up taking it down to the studs anyway. In my case, my previous tile had been up for 20 years
mastic directly to the drywall and it was perfect. So, I am setting the tile directly on the drywall again. Hope this helps.
-Jefferey Fry ex-tile-setter-helper-in-the-early-70's
Currently general partner of MJD, a diversified construction company. Cali