Too hot is uncontrolled, approaching the boiling point of your coolant. In a "normal" engine I'd put that at somewhere in the 210F to 220F range. My OEM Jeep runs with a 210 t-stat to increase the efficiency. Many of the newer OEM systems are red-lined in the 260F range. If your system is sized with plenty of radiator to dissipate heat, and plenty of water pump volume to keep coolant circulating then increasing heat increases your thermal efficiency up to the limits of the coolant you choose to run. Just my $0.02..