Even though they might follow the same scheme, the tradeoffs for each level may be very different. Say, 9 vs 5 for zlib may mean going only 2x slower and using the same amount of ram, while for lzma it may be 4x slower and using 3x more ram. Therefore copy-pasting instructions from some forum where the original author happily used zlib compression with level 9 and using it on a repo compressed with lzma might result in quite suboptimal results. Also, even though the compressors you've listed may follow a standard, there may exist others which don't. Therefore I still think it's better for each compressor to have its own compression level flags.