I investigated the zlib C interface, and there is no way to extract those fields which is unfortunate, so we'd have to cache them. The type of compression fundamentally is changed based on the value of those fields, so if it's not much overhead, I suggest adding ivars for storing it... that being said, it might be a pain.
inspect ideally would print that data. It could even be nice if it printed a compression / decompression ratio, but to be honest, that might be stretching it a bit.