Mustang dyno's are great tuning tools and are heart breakers on the numbers. They are about 10-12% less then a dyno jet. That doesnt mean the car is making less power, its just reading it differant.
upper 11's - low 12's and 5 degrees of knock retard on a stock car at WOT is pretty normal. Most dyno operators use the tail pipe for a O2 reading unless the car has a header bung.
If you tune out the tail pipe and depending on which wideband you use, it can be a 1/4 to 1/2 point richer at the motor. Just know your equipment.
These motors really like the 12.4 to 12.7 AFR range for best power.
Phil
www.TampaTuning.com