I always find it strange that dynos do not seem to be calibrated in some way, and then from that the true bhp could be worked out with built in allowances for air temp, humidity etc.
If your guy says his dyno under reads how does he know and by how much does it under read?
I am lucky in having two dynos within 5 miles of where I live so before the winter comes I am going to have mine put on the dyno at Jordans just for a power run to see what the difference is between their dyno and the one at RaceHQ where I had my custom map done.
It will be interesting to see what the difference in dynos and air temp and humidity make.