On the topic of underrated cars the 89 turbo trans am gets my vote. Aside from the ZR1 corvette, it was the fastest GM production car until the 97 C5 came out. 01-02 LS1 Camaro's usually put down ~300-315 to the wheels, which is what they rated for at the crank.

Second to the 89TTA, I'll also throw in the Syclone and Typhoon for second place.