I bought a used 2001 GT with 37k on it. In a year and a half I have 86k on it now. I drove it easy after changing to Bosch quad 4 plugs and averaged 29.7mpg over 19 tanks of gas. I work 102mi from home, 98% highway. To get the good milage I need to keep it at 70mph.. other wise the mpg drops to 24/27 mpg. My best tank so far was 31.9 and backed it up with 31.4mpg.
It does have a few squeaks.. door panels mostly, but rides great. Burned up the stock Goodyears at 85k and switched to Michelin, but now my milage has dropped.
I know of a few guys that run the GTP and pumped them up to around 450hp.. not sure what they did, but they do visit the transmission shop often...
I would buy another GT, a lease turn in is the cheapest way to get one..