Maybe if you post up some of your time slips so we can get a better idea.
Also what are you shifting at and what gears do you have?
2016 VW GTI 4 door DSG 1989 Sonic Blue Mustang LX Coupe, 349 ci, 392 rwhp 424 rwtq....Mustang Dyno "There are three kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves." -Will Rogers-
OP it depends on the altitude of the track and the air density. I assume you are on stock Pirelli snow tires. The best time from Motor Trend was a 13.5 and of course that is a SAE corrected time; if the times were not corrected tests on different days and tracks would be meaningless.
My best time for my pure/bone stock 2009 GT was 14.021 @ 101.04 at a density altitude of 1320 feet above sea level corrected to 13.823 seconds @ 102.513 MPH.
If you saved our time slips you can go to Drag times and get the corrected time. Be warned if you were running at sea level on a perfect day the correction could actually ADD time to your run. DA Calculator - Density Altitude Calculator - DragTimes.com
If you have forced air air-density doesn't mean as much as super chargers make their own weather.
Run with about 1/4 tank of gas and no spare; take any and all junk out of the trunk. 1/4 tank and no spare cuts .1 off my 1/4 time; this is assuming a perfect run and not having to look for third gear.
I now have a Hurst short throw and Ford Racing CAI and tune, 13s all day long even after I eat the free hot dogs on Wednesday family night.
BTW this is with no track prep as these are not hard core big money events.
2009 Mustang GT Deluxe, fake hood scoop, Ford Racing Bullitt cold air intake and tune, Hurst billet competition plus shifter.
Suggest setting the rpm limit at 6800 rpm with the tuner, which will allow you to make the 1-2 and 2-3 shifts at 6500 rpm. This enables you to cross the finish line in 3rd with your stock tires and 3.55 gears, which should lower your e.t. as it prevents loss of momentum with the 3-4 shift.