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Would either option be safe for an 18-foot Kevlar canoe traveling at 80 mph without an additional bow or stern strap?

I believe in the old adage " Better safe than sorry"  ;)
I would have 2 bowline ropes on the front and rear as well as the straps on both cross racks.
You don't want the canoe to slide forward or back if the cross rack straps should loosen for any reason.

I'd get the cam-lock straps for the cross racks rather than using a rope.

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I've even seen canoes with a steel cable around both cross racks and around the yoke with a padlock so know one can steal it without a lot of effort.

Something like this :

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