I have not tried a speed range that low but I may have to in the future. FWIW, I pulse up to 28 miles per hour and glide back to 20-21 miles per hour. I learned of this from VaBeachPrius. With this technique I have acheived a personal record of 152 mpg on a 7+ mile segment from my bank to backing into my garage. I will sometimes let my speed drop below my target range if I'm cresting a hill that has a downhill that is big enough to get me back into my desired range. Otherwise I will try to at least be at the top of my range when I get to the bottom of the hill. When pulsing back up to 28 miles per hour, I try to keep my rpm's as close to 2K as possible and try to never go above 2200 rpm's per Hobbit's research on acceleration in a Prius. This acceleration rate will show an instant mpg on the FCD of 20-22 mpg. Also, if the battery is high enough, I will extend the glide at 20-21 miles per hour by using battery. I try to start with 5-6 bars and like to have 3-4 bars by the time I get to my driveway which is ~.1 miles long with a 1-2% uphill grade.

Warp stealth, if I understand correctly, is when you are over the 41 miles per hour glide threshold and you let off the pedal and get into EV(yellow arrows only) as you are gliding down to a lower speed. I try to use this technique whenever I can on roads with higher speed limits/traffic.