This paper considers the impact of clustering demands on download performance of data grids. The performance metrics are hit ratios and average access latency. For a replication strategy, we build a mechanism by means of majorization theory to compare system performance for two clustering patterns. We employ proportional replication strategy as an example to illustrate the effectiveness of this mechanism. We find that clustering can increase local site hit ratio of proportional replication strategy. However, clustering does not always bring benefits to system. The inter grid hit ratio and average access latency will deteriorate for some cases. If the function of inter grid hit ratio is convex on the domain of file request rate, clustered demands will increase inter grid hit ratio compared with uniform demand. Simulation results validate the correctness of the trend deduced by this mechanism.