In 16 bit mode you have potentially 2 to the 16th power tones (65,536) instead of 2 to the 8th power tones (256) that is if it is captured at that bit depth. If your capture device is only capable of 8 bit grayscale or 24 bit color (3 channels x 8 bits) then your best bet is to capture the image in color mode, convert ot 16 bit in Photoshop immediately, then use the Channel Mixer to convert the image to Grayscale at 16 bit. THis will give you a rich 16 bit file in grayscale.
Never scan in CMYK mode ... Always scan in RGB 16 bit or Grayscale 16 bit mode. CMYK is used in the printing industry. Each time to convert from RGB to CMYK or in the opposite direction you lose data. Most color work in Photoshop for photographers and when printing to inkjet printers is done in RGB mode. Even though an inkjet printer PRINTS in CMYK mode, if you send a CMYK File to an inkjet printer from Photoshop, it is expecting an RGB file, so the printer driver first converts it BACK to RGB before printing it, so your file gets degraded one more time.
If you are capturing images with a digital camera, then use RAW mode if you have it - that will usually be 16 bit color. Don't use any of the grayscale or sepia settings - they will degrade the image. Just use RAW mode in RGB, or if you do not have a RAW mode, use RGB color at 8bits per channel and convert as mentioned above.
If your camera allows you to also set a color space, use Adobe 1998 as the colorspace instead of Srgb - and do the same in your Photoshop Color Setup.