Recumbent brakes can be awesome; if the CoG is in the right place the braking potential is vast and with the right brakes it can be exploited too. A year or so ago I happened upon a recumbent rider on his new tadpole machine. His previous machine (also with 20" front wheels) had 70mm SA drums, he told me. He'd test ridden discs as well as drums in both 70mm and 90mm sizes and had settled on the 90mm drums for the new machine. IME SA drum brakes take a long time to bed in fully and his brakes were not yet bedded in; despite this the brakes were clearly very powerful indeed.
Just for fun he demonstrated the brakes and I used my (pretty average) brakes on my upright. I stopped in a normal distance and didn't quite manage to throw myself over the bars. He stopped in less than 1/3 the distance without any trouble. In fact, even with the brakes not bedded in yet, he had so much power that he could lift the rear wheel and potentially mash the front mast (and his feet) into the tarmac. Having seen that, I am of the view that on such machines, once bedded in, 70mm SA brakes are probably 'plenty'. Even with those, few other vehicles would be able to pull up as quickly.
With just a single coaster brake on the rear wheel, yer typical Dutch bike will be lucky to manage a 0.25 or 0.35G stop. Most uprights with a front and rear brake will manage a 0.5G stop, maybe more, up to about 0.75G if conditions are favourable. On a good recumbent the braking potential is limited only by tyre grip so ~1.25G with half decent tyres is possible. Better yet if the brakes have a) good cold 'bite' and b) the back end comes up slowly enough (for you to react to it), you can slam the brakes on full bore instantly and this really shortens the stopping distance.
cheers