Photoshop :: RGB Vs CMYK

This is probably a hoary old subject but I've found surprisingly little help via Google, etc. I just finished creating the background for a large map, with over a dozen RGB layers, most with masks, a number of adjustment layers and some alpha channels. I've heard that for CMYK output it's best to work in RGB and convert to CMYK at the last minute, and colour correct in CMYK.  That made sense to me for a number of intuitive reasons, but now I'm writing up the metadata and I want to give factual reasons to do this, not just my assumptions. 

Obviously file size is one.  It took 5 minutes to open or save this RGB file, CMYK could have taken 33% longer. Can anyone tell me how the math is done in PhotoShop?  I'm wondering about the blending of all those layers with different blending modes, transparency, adjustment layers etc.  In my map each pixel must have gone through a dozen or more math operations.    Will RGB, with it's 255 levels, be more accurate than CMYK with its 100 levels, or is the math all done in double precision and rounded at the end, or is the math all done in LAB or some other colour space?

Also, I recall encountering some PhotoShop operations that were greyed out in CMYK.  I tried to discover them today on a test file but drew a blank.  Can anyone think of any? Any other pros and cons to RGB over CMYK? Andy PS: This forum software is unbelievable.  Hard to believe Adobe would have something so clunky. Like they're actively discouraging the use of their forums.