The gist of the paper I’ve linked is probably in this phrase:
Whole point is doing 1 lookup.
In this case POM cost increase for 4 layers, as compared to single layer, constitutes 9 PS arithmetic instructions for each iteration.
My reasoning for going with sharp blend instead of height lerp lies in fact that with height lerp between layers, areas where several texture mix, will have an unusual height, that does not correspond to any of the textures. It looks slightly unnatural. Simpe max blend is also cheaper.
But yep, there are no issues in using height lerp there instead of MAX.
It is not drift free, but at low depth scale it is pretty decent. It is possible to reduce depth at blend regions slightly to further reduce the drift, caused by blend map.
I don’t know of any realistic way to blend more than two layers seamlessly without mutlipass.