It is a big space to cool. Even with my T & C's, the fan was at max speed for a long, long time. It seems contradictory, but the higher the fan speed, the warmer the air coming out of the vent will seem. Try putting it in manual and lowering the fan speed -- does it feel cooler? But it will take longer to cool the entire space down.
Yes, R134a is less "efficient" than the CFC freon refrigerants, but that can be compensated by the design of the system.
linked to my thread, but that was a complete failure, although intermittent. When it was not working, there was no cold air.
We get hot, sticky days here in the NYC area -- 80-90 degree temps with higher humidity. Working properly, the van was comfortable in minutes, although the fan speed might stay at maximum for a long time.
And it may sound silly, but double check the temperature setting for rear zone (and the passenger side...) And checking the vents is a good idea, too.