High-repetition-rate high-power laser systems induce a high average power heat deposition into the gold-coated diffraction gratings. To study the effects of the thermal expansion of in-vacuum Pyrex gratings on the laser properties, we scan the pulse energy and repetition rate of a 200 TW laser system while monitoring the laser wavefront. Through the measured changes in laser divergence and focusability, we define an average power limit below which the in-vacuum compressor can be used with no degradation of the laser focus quality.