Fully agree!
Specious - nifty SAT word!
On bumpy curved roads when trying to put the power down, there is definitely a difference. If the surface is smooth, then there is a comparison. I have autocrossed both IRS and live axle RWD cars. When hitting a tight corner and getting on the power, the feel was similar. Of course, there are cases where you can overload the wheels and get axle hop, but that is another dynamic.
My suspicion is the live axle got the nod definitely because of price since the additional development time and cost to make it work for the GT500 might have come up against deadline and budget limits, respectively.
At any rate, I think we could see an IRS in this car and possibly the next Mustangs, but obviously nothing is certain.