Much of the trouble is you will never simulate this surface with a grayscale height map, whether with bump or with displacement.  The best you can hope for is a cratered surface, and on close inspection it will fail.  The cavities in the surface of a real sponge are very frequently going to be the same X/Y coordinates but have overlapping and different Z surfaces (relative to a given polygon).  Not a math wiz but I don't see any possible way to represent this with a grayscale map of any kind.  I think the Cellular node can probably represent this kind of surface mathematically, but the problem then is how do you shade that onto a 2d surface of polygons?

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