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.
Depending on your body proportions you should go for a longer stem. 1cm longer is a huge difference already and will make you much more aero.