It is early to draw conclusions, but we can have a few observations:
zsync without look-inside closely matches rsync's performance. I should investigate the block size that rsync uses. It seems that zsync's default closely parallels rsync's default on files of the order of megabytes in size ; but for files of hundreds of megabytes, rsync is using a rather larger blocksize. zsync with look-inside is significantly ahead of rsync, and is a clear winner with data compressed without --rsync.
Where available, it appears that zsync does best using the compressed data and the look-inside method.
Look-inside is beneficial, particularly on non---rsync gzip files. There is not yet any evidence that the larger .zsync files it needs, relative to just using gzipped files with --rsync, are ever harmful, and certainly it appears beneficial.
gzip --rsync produces files that work with zsync whether or not look-inside is used. But, in the current version of zsync, it is less effective than zsync's look-inside behaviour. zsync currently does worse in some situations when --rsync is used - but it is possible that zsync could be tuned so that --rsync was, at least, not harmful to its performance. Some of the results for gzip --rsync files look anomalous.