A Sportline upgrade in Asphalt 8 is a high-level performance boost for specific, top-tier cars (such as the Mosler Super GT and Pagani Huayra BC). It acts as a super-upgrade, increasing a car's rank and stats beyond the standard "PRO" level, making it exceptionally powerful.
In order to apply a Sportline upgrade, the car must be fully upgraded (PRO) first.