A longer stem for more control? I would have said a shorter stem would provide more control, thats what I found anyway when i switched to a shorter stem.
Although I wouldn't adjust my stem length for any other reason than comfort (for XC) I can see how more serious riders would to get a more racing fit.
Stem length IS measured center to center.
I found getting the right stem length is just trial and error. I tried 4 different stems before I got the right rise and length.
Sorry couldn't be arsed to use quote function.