The performance of an indirectly cooled cryogenic silicon monochromator under heat loads up to 870 W has been studied. The investigation was performed over numerous parameters and included measurements of total flux, spectral density, rocking curves, angular beam profiles and crystal slope errors. An almost ideal monochromator performance was observed in the 270-570 W range of the heating power. At a heat load of approximately 400 W and under standard operation conditions, the crystal distortions did not exceed 1 micro rad. At the highest available heat load of 870 W, the crystal distortions were about 7 micro rad.