One thing to try is to let someone else drive your car and see if they can improve on your time at all. If not, it's car, if so, unfortunately it's driver
But I think it might be car. What Octain are you running in that thing? How full is the tank when you ran it? I ran 15.4 in a near stock SE with about 5/8 tank of regular gas, so you should be ok if you ran on mostly full tanks.
