Stock 5.0 HO motors idle at 20*... some like more, some like less... If i were you i would let the car idle at night with the hood open and check to see if the headers start to glow..
As far as the guy that tuned the car is concerned maybe he missed it?? but somehow i doubt it.. MS get a bad reputation for things like this... Now imagine you pullied the blower down so that you only saw 10psi max and held in it until you hit the rev limiter, that hole in the fuel table where the numbers go from around 115 then drop to 90 or less is a major issue. That one spot alone would leave you with parts scattered across the track.... thats 30% less fuel in that one spot, so at this load and rpm point you would see 16 AFR at 10psi and 5500-6000rpm....