I normally pull ~4% fuel out when I put a street tuned car on a dynojet (just to make numbers as there is no other reason to really be on a dynojet). A Mustang dyno opened locally recently that has a fully enclosed, climate controlled dyno room, full instrumentation, excellent airflow and user selectable load settings. Tuning heaven but the numbers are a letdown in comparison.
Boost control is really what separates the men from the boys, when used in the classic method of a spring and a normal controller. You either need a stiff spring that is too much for pump gas so you can max the turbo out or a pump gas spring that is going to leave a decent ammount of power on the table. Recently, some have been experimenting with CO2 to hold the wastegate closed past where it would open with a weaker spring. Seems to show promise but, adds complexity and $.
Tial 44s suck for low boost BTW. Very variable based on spring temperature. They also don't have the preload screw some other wastegates have that allows for "cranking up" the spring rate when you want to run more than normal boost. You have to change the spring or add a second one. Not that I dislike Tial. I did go with an even smaller F40 Tial on my 7M, because it will hold more boost than any other gate I know of with the 2 stiffest springs they make jammed in there and the wastegate hose pulled. My car doesn't even turn on with less than 120 MON in the tank though...
On my 2JZ SC300, I found the best dyno for numbers to be the dyno dynamics. I was stuck with the Toyota wastegate spring (stock twins) and could get 23 psi on the street but barely 20 on a dynojet and Mustang it made the boost but shows lower numbers. The local DD was the best compromise for that scenario as it has load but makes numbers close to a dynojet. It made 390 hp on a jet and 447 on dynamics in very similar trim.
Also note that if you put different gas in just for the dyno, that will also affect the A:F based on specific gravity and evaporation rate of the fuel. ECUs can't compensate for this in open loop mode.