It sounds like normal mustang behavior to me, I live in the Charlotte NC area where we have had plenty of rain these last few weeks.
Just have to back off the throttle, like others have mentioned, little to no gas while cornering, wait until you have it straightened out to get on it and you should be fine most of the time, and watch out for the glossy looking water puddles on the highway, just coast with the wheel straight though those.
Even with the 255 Eagle F1 Supercar summer tires on the '13 PP it is managable, by wife drives the car about 50% of the time(we switch nearly daily between the car and our '11 FX4) rain or shine with out any incident.
Yeah all the rain we had this past month was really taking a heavy toll on our roads. Water has been pooled up all over the place, which is probably why this was on my mind more than usual. I think I was spoiled by AWD and this Mustang is the most powerful car I've owned so I just need to adjust to the driving dynamics. I can tell these tires play a huge part in this though...for example:
One road I drive 7 days a week has two very sharp S-curves. Today in 85F degree sunny weather, I could manage about 35mph before the car started to push a little. When the weather is in the 60s like it has been for a week weeks, I'm lucky to push high 20s. I've never owned a set of tires that were so picky about ambient temps, but I'm not complaining. I love the car and how it drives, and I'm really fortunate to daily drive it.
Airaid CAI, Roush axle-backs, Roush high flow upper grill, Raxiom Smoked Projector HIDs