This paper presents high-fidelity experimental traverse measurements downstream of an annular cascade of transonic nozzle guide vanes (NGVs) from a high-pressure (HP) turbine stage. The components are heavily-cooled real engine components from a modern civil gas turbine engine, operated at scaled engine conditions. Tests were conducted in the high technology readiness level (TRL) Engine Component Aerothermal (ECAT) facility at the University of Oxford. High resolution full-area traverse measurements of local kinetic energy (KE) loss coefficient are presented in several axial planes. In particular, we present: circumferential loss coefficient profiles at several radial heights; full-area traverses at three axial planes; and fully mixed-out loss calculations. Analysis of these data gives insight into particular loss structures, overall aerodynamic performance, and wake mixing rates. The effect of exit Mach number on performance is also considered. The data address a gap in the literature for detailed analysis of traverse measurements downstream of HP NGV engine components. Experimental data are compared with steady and unsteady RANS simulations, allowing benchmarking of typical CFD methods for absolute loss prediction of cooled components. There is relatively limited aerodynamic performance data in the literature for heavily cooled NGVs, and this study represents one of the most comprehensive of its type.