I will also point out that there are practical limits to how much of a benefit the sustainer can be for "stretching your glide":

Below a certain altitude, you are unlikely to catch an organized thermal AT ALL. Those super-low-altitude saves happen, but are rare and hard to pull off (and thermalling within a few hundred feet of the ground isn't just about risking a landout; you're also risking a stall/spin death - FES or not).

Above that altitude, you may find lift but you're going to be below the optimum working band so again most of the time when you stretch a glide down into this range the gamble is not going to pay off. Consistently gambling down in this range may help you win one day in a contest; but at the cost of a lower _average_ finish (for all the days that you wind up having to slowly grind your way up from this below-nominal altitude).

--Noel