You think that more ARB strength on the rear is increasing the overall amount of grip you have?
Here is me behind the wheel of a car getting 100mph+ oversteer in a clio by the way if you think im not used to it, lol
http://s7.photobucket.com/albums/y251/chip-3door/clio/?action=view¤t=IMG_0269.mp4
(look at the halfway point of the video)
A stiffer ARB would probably have been enough to have meant that I couldnt catch that and would now have a written off car, lol
If you would like to know a little bit about ARBs you could read this article I wrote a few years ago which covers some of the basics you are missing out on grasping with your "it will improve the grip on any clio" argument currently:
The problem is summed up by your last line TBH, you havent tried one on a car that was setup properly without one in the first place like leeroybrown's is
The standard issue mistake you are making that I see from lots of people just starting to learn about suspension is the "harder = more grip" one.
People drive a car thats too soft (ie most standard cars) and then when its made firmer it gets better so they just assume that the more firmer you keep making it the better it will get, but what happens is you hit a point which is optimum in terms of the roll resistance and then if you increase it further beyond that point it gets worse not better (its a bell curve basically).
So if where Leeroy is currently is the right amount of rear end roll resistance, then adding more will make it worse not better.