Basically all current consumer printers (including wide format) are RGB devices.
If you send CMYK to them the driver will remap this to RGB which goes to the printer which then remaps internally to the inks being used.
So if you send CMYK you are just putting another step in the process which could take longer.
CMYK is generally used in prepress.
Some people I have noticed, think they will get deeper blacks with a CMYK profile to the same printer they normally send RGB files too.
Personally I use RGB only as I can't be fluffed mucking about experimenting, when the printer will only take RGB anyway.
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