Lab Color is Photoshop's true color environment. PS ' thinks' in Lab color,
so what you see is really what you get, depending on your monitor
calibration, of course. I've used Lab to do sharpening procedures, but
rather than using Unsharp Mask, I use the Curves pallete in the L channel
and boost my contrasts, then go back to RGB or CMYK. It is way better that
using Brightness/Contrast in the Adjust menu.
I'm sure unsharp mask would work fine without whacking out your colors--but
be sure to stay in L channel, and not go into A or B, as these are the color
channels. L is only the Grayscale map, so color is not really affected in
L. You can do pretty wild things in the A and B channels, tho, so its fun
to use Curves to get really skewed color effects there.
Going to CMYK is always disappointing to me--the colors always look slightly
wilted to me after the switch!