I have the Rx gain control set at about the 45dB point on the slider
Having the Rx gain slider at around 3/4 setting is not excessive and I doubt that change will make a significant difference. We find some users have very high or low level audio and expect the slider to compensate but it can only do so much, its range is probably too big but for those with no other controls it serves a purpose.
You can test your subjective observations more objectively by having both WSJT-X v1.6 and v1.5 installed. Set v1.6 to save periods with decodes and read the resulting .WAV file into the v1.5 version to compare the decode performance.
