Test drove an '03 GT - SLOW AS A DOG - what gives?? 87 octane?

I test drove a very nice 2003 GT yesterday at a local dealer. It was a 5 speed and was all original. At one point I rolled out from a stop up to about 15 MPH and then jumped on it. It crawled away like a V6 without even a hint of getting close to breaking the tires loose.

What gives? Anyone ever run 87 octane in their GT? I'm thinking the crappy dealer gas has the car pulling the timing way back. Otherwise I'm very sad.

I haven't had a Mustang since I sold my '89 several years ago. I was expecting similar performance.