I decided about a year ago to always run in "power mode" and haven't looked back. You get quicker shifting in power mode and my gas mileage has maintained the same 14-17 MPG. When you drive at highway speed the transmission will still shift to OD & lower the RPMs to normal until you start accelerating it will downshift to third gear pretty quickly.
When I was in "normal mode," The transmission always felt like it was trying to shift up constantly & too early where I would lose the power ban going up hills. It may just be my VX, Colorado altitude or larger wheels but my VX transmission feels much smoother on power mode.