WSPR - how QRP can you go?

It is well known that some people use low milliwatts or even microwatts with WSPR. My antenna is not the best on 10m, but certainly with Es I could run low milliwatts or even microwatts and be copied at good ranges when the band is open. Some of yesterday's reports were plus dB S/N even with 500mW, implying there is 20-30dB in hand.