I'm gonna see if the Xsplit Audio Mixer Extension is any help.
Er, no... It's now built in to Xsplit. The audio mixer in Xsplit has plenty of options and sliders, but jacking them up does exactly nothing - for reasons I don't quite understand. Why have a slider if it doesn't do anything?!
However, I did re-discover the Per Scene Audio extension, which we have installed... It has its own mic volume control. When I turned that up to 100% (from 50%), the audio now hits 0dB (red bars on audio meter) when speaking. So I'm hoping that extension's control was overriding the built-in one.
We'll see if that does it soon enough!