At the moment, high quality excavators are worse at creating canals than low quality excavators, since they take longer to clear the ore patches that must be cleared to excavate land.

I agree, however unfortunately quality effects aren't modable at the moment. Since the excavator is fundamentally a mining drill, it gets the quality effects all mining drills have: reduce resource drain.
It's a much discussed gap in the modding API over at the modding discord so the devs are aware and we can only hope Wube addresses this in a future update. But until then, you'd have to do with keeping the excavators at the lowest quality.