Omnidirectional media formats, particularly 360° videos with spatial audio, provide new immersive experiences and introduce a novel dimension to content consumption. We explore the relationship between subjective data quality and metric performance evaluation in the context of Omnidirectional videos with spatial audio. While methodologies for 360° video quality assessment have been standardized and well-documented, previous efforts primarily focus on video with limited audio conditions, e.g., mono/stereo rendering. Moreover, the experimental test setup and subjective test methodologies impact data quality and the ability to use these data for objective quality metrics performance evaluation. Such a problem is key in the industry and the standardization activities, as codecs and quality models must be compared. Hence, the requirements on the ground truth data quality have to be clarified to allow proper conclusions. In this paper, we compare two setups and three test methodologies to study how experiment discriminability changes with conditions and participant number. Then, we show how discriminability impacts the resolving power of quality metrics. We show that higher-performing metrics require higher-quality data to reveal their full potential. In doing so, we put into relation the experimental cost, data quality, and resolving power.