Blast Freezer

I think your client should hire someone who knows refrigeration systems. Your guage readings indicate that the system is working, something that is obvious since cold air is coming out. Whether it is working to its design point is an entirely different matter, and is something that can only be determined by reading the manuals or calling the manufacturer.

Adding Freon to a system that's already at its design point will not make it any colder and will most likely make it warmer, and consume more energy moving the additional Freon against a higher head pressure.

Also when your client says "improve efficiency" I believe he means "reduce energy consumption". Although related they are not the same; i.e., you can have a system that does more work (makes things colder) at a lower per unit energy consumption, but you still have to pay for the additional energy for the additional cooling effect.

There is no free lunch, you'll need more evaporator and/or condenser surface area to pick up and reject the additional heat required for a greater temperature difference. Hire an expert.

Do you have any pressure readings? What type of "Freon^®"? Does the system ever run in partial vacuum? Is there a purger? Etc., etc.

freon is 404a, low side is -30, high side is 250. no partial vacuum, no purge.

How did the call to the equipment manufacturer go?

The "-30" can't be a realistic pressure.

You need to gather all the data before you can diagnose the system. You need to take superheat and subcooling reading in order to verify the system is low on charge.