My own data point comes from years ago when I was running significant distances. Dropping down 20lbs essentially zapped my "reserve gas tank" (stamina) and impeded my ability to recover. Was measurably quicker, though only out to certain distances. Couldn't maintain the pace beyond that, as the reserves had tanked. Getting back up to 20-25lbs heavier meant I lost a little bit of speed, but I could maintain that speed for two to three times the distance as compared to the lighter version of me.
In that case, I definitely found the lower limit of an effective performance weight for me, at least for running.
Can't say that I've noticed weight differences altered much in my cycling performance. By far the more important change for me has been injuries (pre- and post-), where obvious differences in power output and stamina existed.