Respectfully, I think your right foot is a tad too heavy.
Any powerful RWD car will slide around in the rain if punched hard enough, particularly if you turn the traction control off. The Model S is very powerful, and its instant torque plays into that as well.
Just because the car is the pinnacle of the cutting edge, it can't break the laws of physics. Treat that power and torque with respect, particularly when conditions are not ideal. Do not have the expectation that you can drive the same in the rain as in the dry.
If you are in a line of traffic at a stop light behind several cars, and you accelerate at the same rate as the Honda Civic in front of you and the traction control lights up, there's something wrong. But if you're on your own accelerating "normally" in the rain and the traction control lights up, my first guess is that you're pushing too hard. (In my opinion, "normal" acceleration is what a typical $30k car can do without being in a rush, say 0-60 in about 8-10 seconds. Anything faster than that is above normal. Not saying bad, just trying to set a reference to compare by.)
We have a P85, and its been on snow tires since November. The car is powerful enough to break those tires lose on dry pavement before the traction control kicks in. Its even easier in the rain, and obviously the snow. I have not floored the car since November, and I'm looking forward to the spring when I can put my 21" summer tires back on.
Additionally, if you're used to accelerating fast enough in the dry to trigger the traction control in the wet, you're going to chew through tires really quickly!
