Palgia,
You are quite correct in deviateing from the best glide speed will reduce your range.
I have had flying in ground effect demostrated to me by my instructor at the time in a glider and flying at very low level the speed pegged itself at 87knots and showed no sign of slowing. Suspect it has something to do with the induced drag disappearing. I was told that some pilots do it to recover back to a airfield if they are low on height and need to cover a extra few miles. I modern glider (very low profile drag) can get up to over a hundred knots diving from about 300ft!
Could a Pup do that? I would'nt do in a spam can that is for sure. What would happen if you stalled trying to get over the obstacle?