By no means is it invincible. I've felt the tires let go at extreme conditions and admit I was taken by surprise, but then I learned the limits of stock tires on stock suspension and still realize that those limits are beyond those of most RWD cars.
I've had RWD cars that handle very well even in bad conditions (GTO), some that are so-so (Camaro), and some that would step out when you least expected it (Mustang GT). My preference and the one most suited to keeping me safe given my driving style and attitude is my beloved Subaru.
And speaking of a WRX wanting to upgrade to 400whp, I just got new gears (MFactory) and a new Exedy clutch done. Also got the STi shifter/Kartboy bushings. Whole new car....I'm not shooting for quite that much power, but it should potentially handle it a lot better than possible before. Once again not invincible.