^very true. I had no issues with full stiff in the rear until I added a cage at Road Atlanta. Then when going to AMP which is all down hill off camber turns it threw me for a loop. I guess I took it for granted. More to learn.

check you shock settings...

Just saying something is wrong if someone needs full firm on larger aftermarket rear swaybar. I'd be looking at every element to assure that wasn't something in my equation.

If we're speaking in general then I disagree, especially if you don't have a wide array of spring rates/dampers/alignment tools/tire sizes/etc at the track which I am sure none of us do. Sometimes you have to stiffen up the rear sway to compensate for other parts of the setup or even conditions. A super bumpy track with a lot of tight turns might be an example. You wouldn't necessarily want really stiff spring rates yet you would want to stiffen up the rear to rotate through the low speed corners. Keep in mind I am talking in terms of using off-the-shelf parts not fully adjustable fully custom real race car parts. Sometimes we have to make do with what we've got access to which isn't always the ideal situation.

Now if we're talking about the OP's case in particular then I don't think he needs to be full firm on the rear sway at all, I think that is the problem, or a large part of it. Toe issues under load would likely feel more squirmy and unpredictable (like this :driving:) or even result in snap oversteer and not a gradual predictable mid-corner/exit slide. A broken shock would also not be a gradual and predictable loss of traction plus handling would be different in right handers vs. left handers and it would also be felt under braking as the broken side would not be controlling the spring. Shocks settings is the next likely issue but I still still think its a combo of the rear sway being too stiff and application of the throttle being too early which is breaking the rear traction. It's the fact that the oversteer is gradual and predictable that makes me think its handling balance/driving related instead of a mechanical issue.

Anyway, just my two cents. Hard to say anything for sure without data or driving it myself.

We can agree to disagree. I would argue your baseline setup is flawed if you need to go full firm in rear. Increasing roll stiffness in rear on our cars is big no no. He'll I run 1000 lb fronts and 450 lb rears with stock rear bar. My car still turns well enough that I don't have to touch a thing in ax. Obviously JRZs give me tons of adjustment. I just increase front bump for track. But again with way our suspension dynamics work if you find full firm in rear is only way to turn you haven't set baseline up properly. Believe me I have had top teams work with me in analyzing setup and our cars already have issue getting power down increasing rear bar is just going to lift wheel easier etc.