I suspect that the key to distance when pushing off is the amount of weight you are wearing - the heavier you are, the further you glide. Using the same neck weight as Bevan, I was able to complete a comparable push off distance of around 12m. With no weight, went about 9m, so Will you should be able to get close to 15 if heavily wieghted. Would assume that this will hold for stroke distance as well.
Was experimenting this morning with a double dolphin kick in DNF. In other words, push off from the wall, arm stroke, glide, breathstroke kick, glide, dolphin kick, glide and then another dolphin kick in correlation with the arm stroke. Using this technique I was able to complete 25m using three arm strokes, whereas before, using one dolphin kick I was doing it in around five. Bevan, may be worth incorporating this extra dolphin kick in correlation with your arm stroke. The trick is to allow you feet to drift slightly upwards after the breathstroke kick, and then to flex your whole body as you execute the arm stroke, not unlike a monofin action.