I’m not sure anybody knows exactly how ACX calculates RMS values. It’s not a calculation for the whole file but an average (or RMS) of the RMS values of time segments. (Audacity’s calculation may not match exactly).
And when you compress to MP3 the levels usually change a bit too.
The Audiobook Mastering Macro should put you more in the middle of the RMS range while limiting the peaks safely below -3dB.
Your noise is going to increase by the same amount you boost the volume but it should still pass.