88 928S4 Air Conditioning Problems

On the above vehicle, I had the A/C compressor change, the system O-rings, and the Freon filter as well. However, the air from the front vent is still not cold as it was originally.
The system was also converted from R-12? Freon to R134A? Freon.
The shop says the issue may be the front evaporator and suspects its clogged and not allow full free flow. At high RPMs the A/C gets cold but previous, I could not set A/C to lowest setting in mid-summer as I would freeze.
Any ideas? Is there anything else to consider that might be an issue besides the evaporator? What is a reasonable cost for the evaporator? And replacement?

Thank you!
Darrell

Something else to consider is the valve on the heater hot water supply - it is vacuum actuated to close IIRC. The default is is open, sending hot water to the heater core which is hard for the AC to counter. The valve is behind the engine near the firewall in the engine bay, and has an obvious arm connected to the vacuum actuator. You can check if it is seized and/or does not move when the car is started and the heater control is set to cold.