You should return the tune to stock, that will help rule out a bad 02 sensor. You may get a CEL but no big deal. I can't remember if you have catted downpipes or not. In the stock tune 1.0 lambda is set for 14.069 AFR. So if you were logging and reading from the stock 02 sensor the 14 ish you were seeing in just fine.
You really need to have a full compliment of PID's on a WOT log to properly diagnose anything you suspect with the car.
There are so many torque limiters, airflow limiters, wastegate limiters, fuel limiters in the tune. It will be nothing like the Subaru. Your tuner guy was probably just trying to put it in terms that made sense to you. I had a WRX that I did open source tuning on and it was fairly easy to adjust wastegate duty cycle. The SHO uses a torque based approach and is wildly different than a mustang which uses a similar ECU and processor.
You should also be logging actual throttle angle in your WOT logs. Often times when your boost gets a little too high the throttle will close slightly to maintain a certain torque level, you can usually see it in a boost gauge. But when the throttle closes to maintain the torque level it could feel like a miss or hesitation, I could see that
With the stock tune the car actually uses the throttle angle to control the torque. Rarely is the throttle open completely when you floor it.
And btw if you do log actual throttle angle the max angle will be 82
