I'm not 100% sure about the IMAC and the judges discretion deal...but I can see why. It's no secret anyone with a computer radio can literally pass any dB test if they know ahead of time it will be conducted at engine run up and not during actual flying.(*)

Our club has a set rule and if they tried to ban a $4000 plane from a $49 instrument I'm sure some sort of "discussion" would thus ensue. The last time they actually got it out some guys got ticked that their "quiet sounding" to them engines read louder on the meter than some of the more "annoying" ones...well that's the way it works...so it hasn't been out since.

*- don't know if people still do it, but people use to put a "step" on the throttle and when the guy said "go to full throttle" in the pits to check the sound you pushed the stick forward and actually went to 75% or whatever you needed to get under the sound limit. Soon as that was done you flipped a switch and bingo you actually go to full throttle at full stick again.