At WOT, the air/fuel ratio is a function of fuel pressure, RPM, airflow through the MAF (mass air cars only) and the TPS. The O2 sensors don't have any control input into the computer's program at WOT.
Therefore you can use the SOP meter...
Get one of your buddies to ride along with you. Find a level stretch of road where you can do some testing. Watch for the wind speed and direction, it can have a definite impact on your test results.
Don't choose a speed range that will make you shift gears, the shift quality & speed will affect your results. Use 2nd or 3rd gear and do a run from 2000 RPM to 5800 RPM. Call out the RPM as it increases and have you buddy record it and the elapsed time. If you have one of those cheap digital watches like I do, a stop watch is built in.
Start the fuel pressure at 36 PSI, vacuum off and make a run. Turn it up to 37 and make another. Be sure to reconnect the vacuum every time you set the pressure. Keep turning up the pressure and making runs until you notice a drop off in time. If your bud was careful to record the results, you'll have a nice chart to use to find the optimum pressure.