It really depends upon the motherboard and its BIOS. Some motherboards will automatically invoke the XMP settings and attempt to use them. Other motherboards will default to the maximum settings supported by the processor. Still other motherboards will default to the lowest settings (2133MHz). Most motherboards also provide a back-off capability so that, if a setting doesn't work (causes bus hang), the BIOS watchdog recovers and automatically drops to lower settings. This doesn't always work, of course, depending upon how the failure will manifest.
I remind you that using memory and XMP settings that take the speed above the maximum supported by the processor is considered a form of overclocking and could disaffect the lifetime of the processor and even void your warranty.
Hope this helps,
...S
