Fit changes should be done gradually, allowing your body to get used to the new position.

A 10 mm difference IMO would not be much change. As the previous poster noted, 20 mm is enough to have a significant effect. I had the same problem (hand numbness, neck pain) on my race bike with the stock 100 mm stem. Swapping it out to a 120 mm made rides over 2 hrs better.

Dropping (slamming) the is good if you want to get into a more aero position.