I suppose that could be the case if riding in Turbo mode AND you have Mission Control set to discharge the Range Extender battery first. When the battery discharges (below 20%) it starts to cut back on assist and eventually limits to Eco levels of assist. Using just the internal battery, should be able to go Turbo for the first 50% discharge with no noticeable loss in assist.What have you heard about the range extender? It gives less than 50% more range? It's 50% of the size of the main battery isn't it?
I have a range extender and ride mostly in 30/100 eco like assist levels with the Range Extender set to discharge first. I can really feel the drop in assist levels when the Range Extender is at the lower charge levels if it happens and I happen to be climbing. For sure when in Turbo mode at low voltage levels the reduced assist is very noticeable.