Complex sentence: Semi-trailing arm IRS became standard on high-end luxury and performance models (Calais & SS models) and an optional extra on lower-end models, in lieu of the live rear axle, drastically improving ride and handling.
Split into simpler sentences: Semi-trailing arm IRS became standard on high-end luxury and performance models, those being the Calais and SS specification levels, and became an optional extra on lower-end models. In lieu of the live rear axle, drastically improving ride and handling.