We should consider using pigz for the gzip compressed Wikidata dumps (both json and ttl).
Pigz uses Google's Zopfli code internally, thus leading to approximately 7-8% less output file size than gzip -9 (pigz -11, tested with a small testwikidata JSON dump).
Given we compress the data stream during dump creation, it doesn't matter much that Zopfli is quite a bit slower than plain gzip.