I am not an engineer, so totaly understanding the specs and charts in the PDF is a struggle for me, but this quote from page AC 28 of the PDF seems to confirm my impression.......

"...To decrease the cabin temperature to below 25°C
(77°F), the MAX COLD temperature (18°C (64.4°F))
must be selected or ECO mode control must be
canceled by turning the ECO switch OFF..."

I assume the reference to 64.4°F is the "LO" setting on our US TCH, which we all know turns off the ECO mode.

I guess an additonal question I have, is this...Does the ECO mode do anything while on a long distance high speed drive, (highway) when the ICE isn't likely to shut off anyway?

From last page of PDF "...This prevents the SOC (state of
charge) of the HV battery from decreasing and
increases the amount of engine OFF time,
improving the fuel economy."

I hope someone can clear this up for us, as this pertains to the original question of this thread...what effect on mpg does the ECO switch have? This may depend if we are talking about in town or on the highway.