Temp will continue to rise 10-15 degrees after pulling from the grill, while resting. If she wants well done, pull 'em at about 150 internal, and rest 10 minutes. Final temp should be about right.
If the charcoal bins were full, coals may have been only 1-2 inches below steaks - really hot. Use less coals, or dump the bins to spread them out, to get a few more inches clearance.
Also, burning charcoal is a hot intense dry heat, whereas burning propane is a moist heat. Combustion of hydrocarbon chains produces h2o/water as a byproduct. This will affect cooking as well.
In order to know how wide open the bottom vents are, mark the ring above the ash sweeper handle. Look down in the bowl, and using a Sharpie, beginning with the handle all the way to the left, swing it the right to just when the vent begins to open, and mark "Closed" or "C". Continue sweeping right, and mark "Open" or "O" for when it is fully open. This will be a range of about 1 3/4-2 inches, and you can also mark 1/4, 1/2, 3/4 increments as well.
Finally, your opinion of the Performer? Was ignition/prep as easy as gas, but with all the flavor of charcoal?