my biggest concern is RWD vs AWD. I drive with purpose and push my car to the limit while no one is around, such as taking highway on/off ramps, wet/snow driving, accident avoidance, getting away from the road rage drivers, etc.
Tesla's AWD instant torque along with almost zero unnecessary understeer/oversteer will save me from going into the ditch vs. I will have to be extra careful when I drive IS500 RWD and try not to loose control of the rear.

 

With appropriate tires for conditions, you should be fine to drive RWD. Just leave traction control on if in doubt!
If it's anything like the IS 350, it'll be tuned for safe understeer, with some oversteer possible at low speed.

 

Have you test driven an M3P? Its quite different than most other cars. It will have far more horsepower and capability than an IS500 coming out of a corner or accelerating. Its pushing 520hp to the wheels so not surprising but its almost too easy to harness that horsepower. There is no drama between foot to floor and the car scooting ahead. Soon as you think it, it happens and that instant response is magnitudes faster than an ICE. But if you are after that old school RWD feel, then the IS500 will probably be the perfect car for you. I think most buyers won't cross shop the two as the experience is completely different even though they are comparatively priced. I think going for a test drive an RCF and M3P right now will be the best indication on which car you will prefer. Have fun

 

The Model 3 Performance and the IS500 are two very different cars.
One is traditional old school, while the other the pinnacle in design.
Test drive both back to back and choose whatever you like really.