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.
2013 V6 Base Performance Pack - Stock..... for now
2012 V6 Premium - GT front swap..... more to come