I'm late to the thread, but I've been extremely pleased with my Schwalbe Marathon Supreme 20 x 1.6 tires on my Bike Friday Haul-a-Day in the three months I've had it. Sadly it looks like they are being discontinued in that size, but they're still available from some web-sites: 20 x 1.6 Marathon Supreme (Folding)

I was initially worried that they were too narrow; but my bike, plus kids, plus me is over 300 lbs and these tires provide a stable, smooth and fast ride. While I haven't had an opportunity to test their durability, I have no worries about their longevity after my positive experience with the Schwalbe Fat Franks on my Fr8 cargo bike (after 7 years and a lot of miles and abuse, the tread finally wore out on one tire).

Also, I know lots of people with Xtracycles Edgerunners and Haul-a-Days who regularly carry loads upwards of 300 lbs and I've never heard anyone complain that their tires, stock or otherwise, have worn down or blown out so I don't think your experience is common.