Note that the high-rez square primitive prop tends to make slightly distorted reflections; a large number of polygons on the reflector does not improve the reflection, so there is no reason not to use a single sided square prop, and it will render faster as a bonus.  Hope this helps.

ps: that is with 1 raytrace bounce, 1 light.  more bounces and lights can improve quality, this was just a minimal test.

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