Hmmm, that makes a lot of sense, I think at those loads the chassis flex really comes into play, I took this from the Awesomatix pan car product page: Additionally, the alloy side braces on the outer spring frame offer several possibilities to play with rate of the “rear suspension” by simply moving and adjusting a set screw. The stiffness of the chassis can be adjusted very quickly for a variety of track conditions.

This leads me to suspect that under very heavy load, the chassis is flexing as such that it changes the load between the inside and outside tires in a way that makes it more forgiving to drive, hence it being a setup option as people can probably find more time with a stiffer ride with the tradeoff being drivability.

I wish I had a 1:12th car, so I could give a bit more real life input, but maybe some of the setup wizards in this thread can take a look at the video you posted and see if they can see any issues.

Goodluck with the mod, and I hope to try it in the future!