The Shimano BR-R650 caliper brake uses the Super SLR system. I believe Super SLR relates to the amount of cable pull necessary to activate the brake. Does anyone know if the caliper is compatible with Paul Component's Canti Levers which are short pull brake levers?
Super SLR mismatches in this direction result in mushy feel and needing to get the gap between the pads and rim smaller than normal to avoid bottoming out the lever. You'll get brake rub from wheel flex and out of trueness that would otherwise be negligible. It's a lesser version of the problems you'd have putting a v-brake on a short pull lever. It's enough to notice but not so bad it doesn't work. You'll have to live with having taken two nice components and made a mediocre system out of them. It will feel and perform way better if you just get a Tektro brake of the same reach and call it a day.
I think the various versions of SLR 'technology' only incrementally changed the cable pull and made the lever pull non-linear - presumably with an increasing mechanical advantage as the levers are actuated.
In general short pull levers are compatible with road bike caliper brakes, so your canti levers should work. Impossible to predict if you will get the feel that you want though.