Now that I have my K2 connected again for computer control I have found that a few things in KComm that worked with the K3 don't work with the K2 because the control commands, though they may look the same, don't all work the same way on both radios. So I spent yesterday evening fixing the problems.
One of the things that didn't work was the auto-repeat option for CQ calling. I was testing it by sending a CQ on 30m with just 1W output into the magnetic loop in the attic. I didn't expect anyone to come back to me, and no-one did, but I was surprised that my signal was spotted in Northern Spain by EA1GFY. The effectiveness of that magnetic loop antenna never fails to amaze me.
I don't think many people use as little as 1W on PSK31 but it would be interesting to see what you could work with such low power. It seems to me that 1000 miles per watt should be perfectly achievable. I've made a few contacts using the K2 and 4 or 5 watts over similar distances to what I'd expect using the K3 and 40 watts. I think conditions, more than power, determine how far you can work. More power just makes it easier.