Nope, I've just changed mine to 7860 * 800 no problem...
Are you using 1920*1080 media as well - it may be downscaling? Not sure....
If you mix a 1920*1080 video with a 7860 * 800 and the combing video method is "scale smallest image (faster)" then it will downscale to 1920*1080.
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