Adaptation of the Modulation and Coding Scheme (MCS) within the Cellular Vehicle-To-Everything (C-V2X) sidelink has the potential for a wide range of applications including congestion control, support of variable packet sizes and improved support of unicast transmissions. However, the practical implementation of MCS adaptation presents a wide range of implications for the C- V2X radio resources, computation of power levels and the operation of the Sensing-Based Semi-Persistent Scheduling (SB-SPS) mechanism. This paper presents the first study that provides a detailed analysis and an imple-mented model highlighting the implications of MCS adaptation on the operation of SB-SPS. This provides the foundation for other applications of MCS adaptation within the C- V2X sidelink. To showcase the use of MCS adaptation, a quantitative evaluation of its performance for distributed congestion control is undertaken, while considering different vehicular densities. The results indicate that MCS adaptation can be useful to reduce channel congestion by decreasing resource occupation, but may not improve the overall packet delivery rate unless subchannel occupation is reduced.