I ran my 2600 with an AIO outside at an ambient temperature of 4°C. Managed to run 4,4Ghz, which gave me a CB 15 score of 1510. I hadn't much time to play around since it started raining then. I hope it gets like -10°C in winter and the coolant doesn't freeze. I think that would give me clocks above 4.5Ghz.