Starting in 2009, the Hinode team has performed a sequence of annual calibration measurements aimed at determining the flat-field response of the Spectro-Polarimeter (SP) of the Solar Optical Telescope (SOT) on the Hinode mission. From these measurements we not only determine the flat-field response for the entire system (telescope, optics, and detector), but using the average disk-center quiet Sun as a standard candle, we determine the history of overall changes in throughput of the instrument over its active lifetime. The throughput has decreased nearly linearly by about 26% between 2006 and the present. This decrease in throughput is accompanied a corresponding increase in the noise present in the polarization measurements. By examining the polarization signature of the spectral continuum in the annual flat-field measurements, we are also able to measure precisely any changes in the intensity-to-polarization crosstalk of the SP over its full scan range. We find an ongoing increase in residual instrumental polarization, particularly in Stokes Q, starting from undetectable levels at the start of the mission to a few ✕ 10-3 (relative to Stokes I) in recent years. This polarization might be explained by degradation of the anti-reflective coatings of the collimator lens.