mozjpeg v1.0 is a fork of libjpeg-turbo with 'jpgcrush' functionality built in.
The 'jpgcrush' feature finds the progressive coding configuration which uses the fewest bits. This most frequently reduces file size by 2-10%, but those are not hard limits. Significantly greater reductions have been observed.
Library configuration defaults are the same as for libjpeg-turbo, in order to make transitions as painless as possible. There are new configuration options for new features, but they are not enabled by default.
The 'cjpeg' program defaults are not the same as for the equivalent program in libjpeg-turbo. The 'cjpeg' defaults for mozjpeg are set to aggressively optimize for smaller file sizes.