Mitch's Challenger is about as dialed in for autocross as you can get a stock suspension mopar. It was built for national level SCCA stock class competition.
Once upon a time , a t-bar mopar set-up was very desirable and could be set up to run very well, as demonstrated by Mitch's Challenger. It always had a compromise with the caster/camber relationship, but that could be aided with spacers on the lower ball joint, shorter strut rods and offset control arm bushings. Now days, the limited choice of t-bar selection has made stock performance set-ups even more challenging to achieve.
The aftermarket coil over suspension do have a huge advantage in spring selections can be changed in 50# increments, and you can find these springs in swap meets in a range of rates for about $50 a pair. They have geometry that is more dialed in, out of the box, than most average guys will ever achieve with stock stuff. They are lighter than a stock system. Also, a R&P system provides precision a recirculating box can only dream of. I won't say they have an advantage in brake access because ARE and Dr Diff have plenty of kits to access big brakes. They also lake the millions of rough road miles that a stock system is capable of achieving. Will some get there, I'm sure some day, but these also aren't daily driver cars anymore either.