You may see slightly colder temps (~1-2 C) going from 13.5V to 15V
IF your water-cooling setup is able to handle the extra hot-side power/heat. I recently tested and reviewed this cooler (MCW5002-64T) and found that over 12V produced minimal drops in temps and I was using an Iwaki pump, (2) 6x6x2 shrouded heater cores in parallel and (4) 120mm Delta SHE fans at 8V...
Heat load curves vs Applied Voltage