With the preamp in front of the FT817 I can copy down to at least -127dBm, which is the lowest the generator will go without external attenuators. Without the preamp, sensitivity is at least 10dB worse. Of course, in reality, external noise will set the real limit. On my FT817, the IPO in/out made little difference to the sensitivity. I suppose the NF was being set by the preamp. It is definitely worth having the preamp (with filtering) in circuit.
I am so grateful to the kind anonymous person who sent me this preamp. Thank you again. I have no idea who you are but I am extremely grateful for your great kindness. There really are some very kind people around.