When in power mode, my Gen4 is faster off the line for only a few hundred feet... but then she goes back to being a Prius.
I drive eco mode all the time and full power does not come until the pedal is almost 2/3 or 3/4 to the floor. Whereas in power mode it starts responding somewhere near the top. Stomp on the gas exactly the same way on either mode, and that pedal distance and the upward resistance of the pedal will make a difference no matter how miniscule. It's just physics.
Well, if you take time pressing the pedal going through all the middle positions, sure there could be a delay getting the WOT. If one does it quickly (like I do it), there is no significant delay, especially considering the electronic throttle puts delay in the throttle reaction anyhow. now, going from eco to power, it could be different, there could be extra electronic throttle movement delay. I don't drive eco, can't stand it.
There's a bit of apples/oranges, going from 3rd to 4th gen: I've read 4th gen ECO is about the same as 3rd gen normal, and so on.
