In-service deterioration can lead to undesired shape variations on high-pressure turbine rotor blades. This can have a significant impact on efficiency, power generation, and component life. Loss of power production from the high-pressure turbine rotor causes engine over-throttling to compensate for the lower performance. This will in turn worsen the operating conditions and ultimately reduce the life of the component. The aim of this study is to provide a high-fidelity flow simulation of in-serviced shrouded high-pressure turbine (HPT) blades of a modern Jet Engine. Shape variation effects on the aerodynamic performance of several shrouded HPT blades with a different number of in-service hours have been investigated. In order to establish a digital model of the shape variation, a novel reverse-engineering procedure is carried out to come up with a parametrized definition of each blade's variations from nominal including any observable damage. The investigation is conducted by means of an in-house, full 3D steady-state Reynolds-averaged Navier–Stokes (RANS) simulation of the flow around a series of damaged rotor blade geometries, which are obtained through high-resolution optical blue-light “Gesellschaft für Optische Messtechnik” (GOM) scans. The analysis shows that the aerodynamic performance of the HPT rotor blades under investigation is primarily sensitive to shroud damage, which is found to account for efficiency losses often greater than 3%, and for more than 80% of the total performance loss. A secondary role on efficiency is found to be played by the blade shape deviation. A highly linear correlation is found between HPT stage efficiency and a combination of shroud damage parameters.