Did the tire installer use the torque wrench for the final 1/2 turn or so or did he tighten them with his impact wrench and then checked the torque with the torque wrench, which is what they often do. In that case I would back the lug nuts off with a 1/2" or 3/4" breaker bar and then see what the impact wrench will do.
I can get the lug nuts off fairly easily with a 1/2" breaker bar on my Axis with 16" wheels and a torque spec of 140 ft lbs. I bet your Porter Cable impact wrench can too if they aren't too tight.
David
__________________
|