Changing the position of your projector, relative to the screen, so that your zoom lens is at full wide angle (projector as close as it can get to the screen) buys you almost exactly 10% more brightness than at mid-point on the zoom (the projector about another 5 feet further back from a 100" diagonal screen). Even going all the way from wide angle to maximum distance back - telephoto, results in a drop of only about 25%.
That is truly excellent for a 2:1 zoom. Many projectors with that much zoom range see a drop of up to 40% not 25%, and on the lower end of the home theater projector spectrum, the Panasonic PT-AR100U ($1199) drops just about 50%! That's a real plus, if you want to mount in the back of your room, such as on a high shelf.
FYI: JVC refers to full brightness as High, and their "eco-mode" as Normal brightess.