Well, at risk of attracting another set of nanny flames...
DON'T DO THIS AT HOME. NOR IN AN AEROPLANE.
OK, that out of the way... I decided to see how much difference airspeed and bank angle make. This time I did it at altitude and measured time for a 360 turn rather than the teardrop manouevre, since all I was interested in was relative altitude loss.
The result was quite a surprise. Holding speed at the hairy edge of a stall does make a BIG difference. I flew each combination at least twice and the results were pretty consistent.
Bank Speed Altitude Loss
45 55 500
45 60 300
45 70 500
60 70 500+
At 55 knots in a 45 degree bank, the wing is most definitely partially stalled. There is a kind of buffet but it comes more from the nose bobbing up and down than from turbulence over the horizontal stab. What really surprised me is the big difference between flying on the brink of a stall, and 10 knots faster. Also that flying in a partial stall is no worse than flying 10 knots faster. That's reassuring from a safety pov.
The 60 degree result is clearly worse. It's also MUCH harder to fly, in my plane anyway - it's so nose heavy that precise speed control gets harder. If I wanted to get really repeatable results I'd have to practise some more, but it's clearly inferior both in terms of altitude loss and certainly in terms of safety, so there's not much point.
My methodology was: climb to altitude (3500'), pull power and slow down to desired speed, enter bank, fly a 360 degree turn holding bank angle and airspeed, roll level anticipating 360 degrees, check altitude. This was all in my 1980 TR182.
The result suggests that even 400' might be doable in my plane, IF you can hold airspeed/pitch precisely. At airports where landing off runway is not too bad (e.g. Livermore where there is a field at the end of the runway), it might be better than the alternative (though not actually at Livermore because the field is long). I don't think I'll be trying this for real close to the ground though.
DON'T DO THIS AT HOME.
OK, let the flames commence...