No, higher power graphic processing unit (GPU) does not improve the performance of ArcGIS Image Server as it is part of ArcGIS Enterprise, and ArcGIS Enterprise is dependent on the Central Processing Unit (CPU). In addition, ArcGIS Image Server is not involved in spatial analysis to solve complex location-oriented problems, which require the use of GPU such as the Slope analyst and Geodesic Viewshed analyst. It provides a distributed computing and storage system, serving extensive collections of imagery, elevation data, rasters, and other remotely sensed data. Refer to ArcGIS Image Server: What is ArcGIS Image Server? for more information. Therefore, ArcGIS Image Server can perform well with only the default integrated GPU available on a machine.
However, GPU processing can be utilized when using deep learning tools. ArcGIS Image Server provides a suite of deep learning tools allowing users to export them to a framework and develop a deep learning model. Refer to ArcGIS Image Server: Configure ArcGIS Image Server for deep learning raster analytics for more information.