An element designed to have 4500 watts at 240 volts will have 5080 watts at 255 volts. But I don’t think this is your problem. Thermostats have to be tight against the tank with insulation and covers in place so the thermostats can react to tank temperature only and not be affected by air temperature. A volt-ohm meter is a must for diagnosis. Since both tanks are showing the same symptoms you would think something external to the WHs would be the problem. But WH control is all internal to the WHs; the circuit supplying power either has power or doesn’t. A 4500 watt element should read 12.8 ohms (please turn power off before checking) across the screws. And with the element unwired and checking ohms to ground (tank) on each screw terminal of the element should read open (0 ohms).