Does this seem like dejavu? :) Not at all, I was already talking about the very similar recompressing of ZIP archives. As a reminder, Slax is compressed by LZMA2 (XZ in squashfs), so uncompressed data is compressed better than already compressed ZIPs. This is the same for gzip, but gnu gzip refuses -0 (level zero) parameter so it basically just can't produce uncompressed output.
So I had to write own software to actually compress using level=0. It was not any hard at all, since I used PHP :) Here is the short PHP script:
It reads data from standard input and writes zero-compressed data on standard output. Actually there are not much gzipped files in Slax, few keyboard mappings and some svgz images, to name a few. Recompressing them using zero compression (and packing later using squashfs) makes entire Slax smaller by about 1MB. Not much, but as always, every byte counts :)