If you really want to make improvements, start with trackday tyres, (if you haven't already) and a track alignment.
Personally, I haven't found body roll to be an issue, and a stiffer front bar will increase understeer.
There's always those back ventilated, kangaroo skin driving gloves to keep the buttons looking good, and you looking like a 1950's British driving God. Handlebar mustache optional.
When you're learning to drive fast, I find it's better to learn in a car that gives good feedback at lower speeds. A good example is Toyota fitting the first 86's with eco tyres. That would slide and rotate like it was doing a rallycross, but gently enough for newbies to catch it.
I wouldn't do...
Sure, the Mustang handling can be improved. Better quality shocks and tying down the rear susp better would help.
But, having had a 300 hp FWD hot hatch, that was a killer on twisty roads, the Ford doesn't give a lot away. I was surprised at the turn in and front grip in the tight stuff, I...
As I said, there are screens, and there are useless hateful abominations of screens needing multiple levels of success before you can drop the temps by 1 degree. And, don't get me started on haptic feedback buttons.
It was probably the glorified tablet display as much as the looks that made me...
I think the S550 interior looks dated, the S650 looks up to the minute. I like the screen interface much better than our Euro cars, it just works better. You can change seat and interior temps by pushing one screen button each, what's hard about that? The rest is on auto setting.
The bottom line...