The engine should run better with a proper working thermostat in place. I can't say cooler but it will certainly get to operating temp more quickly... and that's a good thing, and it will stay closer to the operating temp setpoint... that's generally a good thing.

If you insist on running without a thermostat or if it appeases you mechanic, compromise on a thermostat jammed open.

If you believe that running without a thermostat is good, you might believe that running more than a 50% coolant mix is good. Water is as good a coolant as you'll find. Corrosion inhibitors and lubricants are added to turn water into coolant but the resulting mixture has less heat transfer ability than plain water.

Pull the water pump and check the condition of the impeller. If the cooling system was neglected at some point, the impeller might be corroded to the point of not moving enough coolant even if the pump doesn't leak. Or maybe it was replaced with a low quality replacement with an inefficient impeller.

Consider a product like Redline Water Wetter if you can find it there. You should be able to run a bottle of Water Wetter with 80% water, 20% MB coolant since freezing is not a concern (or even a possibility).

Good luck.

Sixto
95 S420
87 300SDL