Maybe it wasn't the greatest in the looks department, but it did have some punch to it. Stock, in factory trim, the Mustang GT was not able to outperform the Z28 Camaro until the 5.0L came out for 2011. The LS1 is a beast of an engine, period.
Besides, the aerodynamics of the car (yes, laughable, but still) enabled the Z28 to manage 26-ish mpg on the highway.
Having to fill up on premium gas, however, would definitely hurt the pocketbook.