I've got 700x43c GK SK tires on WTB Asym rims. They were a pain to get installed, had to use a compressor.
I too have ridden them down to 20psi and they were fine.
Had a puncture last week and lost a lot of pressure- maybe down to 10psi? It was really soft, so I hit it with my hand pump. For the first few pump strokes there was sealant bubbling from the bead at the bottom of the wheel. That stopped with more pressure.
So, yeah, not that great of a seal at the bead at lower pressures, but I'm not sure I'd want to run those pressures.