I am the one going around you on I-40 in the left lane 5 to 10 over the speed limit. I for one think as long as you are in control a Mustang drives awesome in the rain. That is with both my 2013 with the Summer only P-Zero's and the all season 17's that I have for winter and my 2010 with the All Season 235/50 18 Pirelli P-Zero Nero's.
I have learned to drive to the conditions and do know the limits of my car though.
I did notice that the 235/50 18's did get a little squirrellier in the rain after they had about 25,000 on them on my 2010 and my friend hates his after 17,000 on his 2011 so it might be tires and inexperience with the solid rear axle working together.
Hydroplaning has been pretty prevalent around here lately here in the rain forest, that is just too much speed for the conditions, heck even my F550 has had a little trouble with hydroplaning with all the rain we have had.
BTW where in East Tennessee are you, I am near Knoxville in Clinton and drive to Maryville everyday for my 90 mile commute