My first car did it without the ProBeam and my current car does it with and without. Definitely an improvement with the ProBeam. I have drove an RL with and without the ProBeam...both were far more stable under those conditions.I don't know enough about this to contradict you, I just can't quite grasp the mechanics of it. I assume that the RL didn't have a ProBeam? My NA with a ProBeam was more stable that the RL before I added one to it, and both had what i would describe as a wriggle before I installed it, although it also showed up when transitioning in and out of turns.
Like I said, have no idea whether or not what we were told was true...just what we were told and what I've experienced.