Looking at this application - there are three speakers in parallel....two 8 ohm speakres and one 16 ohm speaker.
Of the two 8 ohm speakers -
Speaker A gets 40% of the total power.
Speaker B gets 40% of the total power.
The 16 ohm speaker, Speaker C get's 20% of the total power.
In this case, the 16 ohm extension speaker will have roughly half as much power as either of the other two 8 ohm speakers.
Relatively speaking, reducing the power to a speaker by half doesn't make a huge difference in volume.