What type of corbels are needed to support a granite counter top?
Well Jeff, the term corbel is new to me also. I always thought corbel was a champagne. But as far as supporting the granite:

I think those corbels are quite ugly and rather than replace them with more ugly corbels, I'd be inclined to build a support structure around the top of the knee wall and trim it out.  If you were to add a perimeter of 2×4's dressed out with 3/4" pine on that wall, you would increase the load bearing area from about 4 1/2" (3 1/2" stud plus 2 × 1/2" drywall sides) to 9 inches. 9 inches should be plenty to support the new 12" top. Add a little decorative molding to hide any exposed 2×4 on the bottom and a little Scotia or 1/4 round along the top under the counter and you got a nice finished look.