Let's go with the equivocal "yes and no". The current programming does not allow EV only above 41mph (Prius II)...the ICE will spin up no matter what above that speed. This is mostly to prevent over reving of MG1 (someone correct me if it's MG2).

Using the EV button the top speed is 34mph before EV mode deactivates. This does NOT mean that the ICE will always start up. It's generally been speculated that the reason for the 34mph threshold is b/c of the marked increase in demands on the battery as speed increases as a reasonable cut off point for the battery's capacity.

So, if you increase the battery capacity, under the current programming, there would be no change in top speed for either EV mode or 'Stealth' mode. In theory, however, if you could reprogram the EV mode controls to allow EV at higher speeds the extra battery capacity would be a necessity to make use of that ability. Still, under current design specs (mechanical), You still could not go over 41mph without the ICE spinning up.

As mentioned, those who've added extra battery capacity have significantly increased the range they can travel in electric mode.

Evan E. Fusco, MD
'04 Prius/BC moded to the T
'06 Highlander Hybrid Limited AWD w/NAV