I did some further tests here just now and was getting FT8 decodes reliably down to -20dB. This seems to be about the threshold for successful decoding here.
This is about 10dB below what I can see on the BATC WebSDR waterfall. SIgnals decoding at -10dB are just visible.
Charlie, I did some tests too, but with JT65A (this mode is selected automatically in WSJT-X dev-1.7) and same results: -20 is the limit , and corresponds to ca. 1 mW (0 dBm) output power in a 60cm wide offset dish indoors.
Perhaps we should try it together some time?