"Even if I get dinged with $20USD in shipping, plus $50CAD in duties and brokerage, I will still come in cheaper (around $295 total with taxes). But then I have to wait for it to arrive, and don't get to support a local business."

As Michel mentioned you only pay, at worst, the same two taxes you pay on the MEC stuff. Now there is a small chance you will get charged duty, because German made panniers are not covered by NAFTA, but there is also the small chance you won't pay the normal sales taxes since they often let parcels slip through, though less so than they did. You won't pay any brokerage if you get them shipped USPS.

Supporting local business is a lauditory goal, but MEC gear is only retailed in Canada, it isn't made here. It may not even be designed here, or the design cost may have long since been paid out, as in the fact they are just copying the seratus design for the new cordura panniers, there are no new canadaina mortage payments getting paid with moeny for that design process. You aren't morally responsible for paying MEC for a decision that was made on the basis of info gleaned from a touring forum, mostly supported by americans.