The Turbo T/A was a high 13sec car. That's certainly not "the fastest" TA ever made by any stretch of the imagination.
Considering it's basically a Grand National engine in a TA, it obviously has potential, but cranking up the boost doesn't keep it stock. People also tend to automatically think 13's or better when they hear "Grand National." The GN's were 15sec cars from the factory in most cases. Turn up the boost, shift kit, and you're in the 13s easy though.