Using a 'wider' zoom setting, and thus having the projector closer to the screen, will produce a brighter picture. This is because the lens is more efficient ( faster ) in it's widest setting. The potential downsides are that this increases the amount of light reflected back from the lens elements into the optical engine ( back scatter ) and reduces ANSI contrast. Also, using 'more' of the lens creates the potential for colour distortion ( chromatic aberration ) and geometric distortion ( barreling).
Using a 'longer' zoom setting, and thus increasing the projector to screen distance, reduces the light output by as much as 40% for longer ( 2x ) zoom lenses. It also reduces back scatter and chance of chromatic aberration. However, using a very small area of the lens can also have potential distortion issues.
Generally, the 'sweet spot' of a zoom lens, where it performs best, is the mid-position of the zoom.
In my experience, the image quality / contrast / distortion changes between full wide and full long are much less apparent than the brightness change.