At 185° the thermostat is wide open. Any more heat and more strain is put on seals and gaskets. While most diesels run more efficiently at hotter temps, about 210° or so, Detroit doesn't like it. And you need to have additional cooling ability so the temps don't get away from you. You can get cavitation damage on the water jacket without an anti-cavitation additive in the cooling solution. Usually not a problem in naturals at designed temps but happens in turbo models.
I ran 6v53 naturals in Vietnam in hot weather and 80° water. Flat out for more than a few minutes and the engines would climb 20-30°. They didn't boil over, but they also didn't go 10,000 hours. More like about 3-5000. The alloys of combustion parts in Detroits have a good temperature range, but high coolant temps also means high exhaust gas temps, and that kills engine life.