Forgot to add, I ran Drumroll in a busy whiteboard at the weekend on 40MHz. The robot has no suppression components fitted. I think every other robot in the fight was on 2.4GHz and I wasn't aware of suffering intereference at any point throughout that fight.

The rules change in 2011 for featherweights so it probably doesn't matter too much but based on this (and I know it's only one case, so not very conclusive) the 2.4GHz units aren't drowning out the 40MHz users.