increased speed into the ground effect, and then using the ground effect to strech the glide can also be an option....
Generally speaking, you're gliding, to make it "over the hedge" to a suitable forced landing area. If the area is flat enough that you could fly in ground effect, you could probably land safely on it there! There is a minor effect of added lift in ground effect, which it momentary if you're gliding. You should be experiencing it during the flare of most landings, so you know what it's like - and it really won't get you anywhere further power off.
On the whole retracting flaps and experiencing an altitude loss.... isn't that mostly a butt feeling? Cause if managing the possible increased speed correctly, and being able to maintain the best glide throughout the retraction, there shouldn't be any loss of altitude?
Aside from a possible engine failure during departure, or approach, why were the flaps extended? Unless the flight manual for your airplane says otherwise, leave the flaps alone as you enter a glide, and extend them only when landing is assured. I would be fascinated to read a power plane flight manual which directs otherwise.
An over riding theme here should be that "best glide" and stretching the glide, and going as far as you can per altitude lost, should not be your primary focus in an engine failure, unless it's simply to make it to the coast, or over the mountains. Arriving to 1/4 mile on final into a tight spot at "best glide speed" is going to demand the most of your skills in power off landing judgement, and the consequences of getting it wrong could be terrible. Better to make a good job into a closer spot if it's available, and have to bleed off speed as you near the landing zone. A judgement error doing that is much less serious.
Of course, there will be occasions where the glide must be optimized to make it to anywhere you could land, so yes, fly at "best glide speed" then. With this in mind, it would be useful to practice power off landings from high final approach at that speed. Demands on skill and precision in the flare will increase doing this!