Every commercial printer in the world uses CMYK. In fact, there is no such thing as RGB printers. RGB is purely an on-screen colour medium. (As someone said upthread, RGB and CMYK use completely opposite methods of dealing with light to produce colour. RGB is additive colour, CMYK is subtractive colour.)
CMYK printed photos are always going to produce a different rendition of colour to RGB – usually 'duller' (especially orange and green), as it is not possible to reproduce the same colour on paper, as it is on screen.
With expensive inkjet printers, many of which use 8 or more inks, it is possible to achieve very good results, even when printing RGB documents on them. But they are still just simulating RGB.
With commercial printers, who print hundreds or thousands of copies of a document on large offset-litho machines, 4 inks (CMYK) is normal. (There is a high-end printing process, called Hexachrome, that uses 2 extra inks – orange and green – as well as CMYK, but it's not cheap).
That's why it is important to convert to CMYK and making any final adjustments on calibrated monitors before sending files to press. And yes, colour profiles and so on can also be important. The best advice I can give, is to always check with the printer before sending any files. They are generally the experts in this case, not the photographer.
