Immersed gratings offer several advantages over conventional gratings: more compact spectrograph designs, and by using standard semiconductor industry techniques, higher diffraction-efficiency and lower stray-light can be achieved. We present the optical tests of the silicon immersed grating demonstrator for the Mid-infrared E-ELT Imager and Spectrograph, METIS. We detail the interferometric tests that were done to measure the wavefront-error and present the results of the throughput and stray-light measurements. We also elaborate on the challenges encountered and lessons learned during the immersed grating demonstrator test campaign that helped us to improve the fabrication processes of the grating patterning on the wafer.