I'd be somewhat concerned about using "water wetter" and that sort of thing.
The cooling system in the E30 and E36 is not perfect, but it can surely work to the extremes of temperature. Big things are ensuring you have the correct thermostat, water pump is serviceable and hoses are in good nic.
Then the fan clutch. If it isn't locking up when standing on a hot day, (i.e. you can stop it with a newspaper very easily, and it takes a while to start spinning again), you've got a problem. Depending on the clutch installed, you may or may not get significant fan noise from standing in traffic in hot weather.
My other suggestion is double-checking the electric auxiliary fan is working, disconnect the wiring connector from that radiator switch, and jumper it (
Per these instructions).
You may wish to use some hot (near boiling) water to double-check whether the switch even works, they're pretty cheap to replace, too.
In my E36, the Owner's Manual recommends a 60:40 mix of water to coolant (or was it vice-versa?). Either way, having a higher mixture of water is perfectly fine, especially if you are using distilled water in the cooling system.
Just remember to use a multimeter to check for electrolysis (Per this guide on
"Doug's Domain") and flush the coolant every one to two years.
Back in February, during the height of the Australian summer, I drove my E36 from the Northwest of Australia down to our capital city here in WA. About 500km/310mi. of the drive occurred in heat that I knew was more than 46º (115ºF), probably exceeding even 50ºC (120ºF). The cabin was stuffed to the ceiling with my stuff, so she was a heavy car. A/C was running, and eventually set to Recirculated. Eventually, I did reduce my speed to about 80km/h, as I was concerned about the hot road surface and the tires.
The only issue I noted was, despite a continuous accelerator position, there was a slight bogging and surging every so often. I attribute this to ECU perhaps not having programming to deal with the sensory input from this excruciatingly hot weather, or attempting to trim the fuel based on input from the O2 sensors. In any case, there was no overheating and no loss of coolant. The electrical auxiliary fan did not go to high speed when I made stops every hour or so just to stretch my legs (etc).
The next day, I finished the last 400km or so of the trip in very mild weather. The car ran better than ever.
All this with a completely stock E36 cooling system; aside from the Aluminium HEPU water pump and aluminium thermostat housing. Car uses a 92ºC (197.6ºF) thermostat, stock 91º/99º radiator temperature sensor.