We also played with (oops...investigated

) our 2011 bot on mecanum wheels. We had a 5 inch bumber height, but we realized that because the bumper is curved on the bottom, that curve aided in sucking in the ball. I believe you need the center of the ball to be higher than (or at least equal to) the midline of the lowest pool noodle in your bumper. This will prevent the curve of the bumper from "collecting" the ball. Unfortunately, it may also prevent you from driving over the midfield bump.
With a little geometry, you can calculate a desired bumper height of 2 and 3/4" maximum. If your bumpers are any higher than this, you will want to look into some sort of deflector on the non-intake sides of your bot.