An additively manufactured ring segment (AMRS) for the first row of a large gas turbine from Siemens was designed with a novel cooling scheme. The wavy rectangular channels used for in-wall cooling are only viable through Additive Manufacturing (AM). Manufacturing deviations have been tracked using an optical 3D measuring system and were accounted for in the calculation models for thermal analysis. Friction factors of the cooling channels have been investigated experimentally allowing accurate flow prediction. The AMRS was tested in an engine at “Full Speed – Full Load” operating condition. It was instrumented with thermocouples and pressure taps. All crucial engine operating conditions were monitored. The results are compared to a conventional ring segment with similar instrumentation. It was installed next to the AMRS in the wake of the same burner. From a thermal model of the conventional ring segment and the measured temperatures the external boundary conditions are scaled to fit the experimental data. These were applied to the thermal model of the AMRS which was used for performance evaluation of the cooling scheme. This work addresses and quantifies the application of AM on advanced gas turbine cooling schemes. The AMRS underwent a mechanical integrity investigation in a referenced publication.