Lowering the stem on a Canyon Endurace I have a Canyon Endurace XL frame with a 110mm stem which I currently have slammed. I've started racing this year and am very comfortable in the drops for the entire race, which makes me think that I might have room to go a little lower. 
Would I be better getting a stem of the same length but with a -17deg angle which would effectively buy me another 21mm to play with? Or should I go for a longer reach? Or both?
It really depends on your body proportions. Canyons have very high head tubes, so -17 degree stem is nothing out of ordinary. Longer stem is a good idea if you feel that the cockpit is too short.

For reach, in my opinion a classic bend bar is better than "ergo" because it has longer reach on the drops, which increases the difference between hoods and drops.