The waveforms tell a large part of the story.
The first cut is conventional stereo, the second is L+R and the third the "quadrature" sum. (Right-click to enlarge)
Supertramp Breakfast In America Stereo, L+R Mono, I+Q Mono
The levels were adjusted for 0 dB in 0 dB out using tone in all three cases.
The L+R level is attenuated by 6 dB; the I+Q attenuated by 3 dB so that they're all the same.
The level differences shown above are how they appear and sum naturally.
The I+Q sum has greater peak level and RMS power than the L+R because it has Side information folded in.
Here is the soundfile with all three cuts in series.
Supertramp "Breakfast in America," Stereo, L+R Mono, I+Q Mono: https://proaudiodesignforum.com/content ... Q_Mono.mp3
The statistics show both a power increase and a peak level increase when the "lost" Side information is recovered.
L+R Mono statistics:
I+Q Mono statistics:
The I+Q sum has about 3 dB more peak level and is about 1 dB hotter in perceived loudness.
The I+Q sum has the balance and "life" of the stereo original - the L+R mono is more restrained because it lacks all of the L-R information.