The FT817 is a bit low in sensitivity on 472kHz so I was expecting a decent improvement with a preamp. To my surprise I am copying no more stations. In fact I am hearing just as many without as with! The result has surprised me. The rig sensitivity is definitely better with the preamp and stations are stronger but it is far from clear cut whether it is worth using it in reality. At the moment my 472kHz transverter does not have the preamp.
Maybe the FT817 sensitivity is good enough. In marginal cases (and these seem few) it probably helps, but most of the time it seems fine without.