Are you sure the transactions you're comparing were posted at exactly the same time?
Your numbers reflect exchange rates of 1.357 and 1.317. That's a 3.5% difference. That seems more likely a result of currency fluctuation than forex spread. The CAD did spike briefly last Thursday in advance of the US Federal Reserve statement on Friday - perhaps that's the difference you saw?
It's a great tip, regardless. Whether it has an impact will depend on the exchange institution. Different companies (and sometimes even different accounts at the same institution) will have different brackets. The $1000 threshold is more likely at banks than discount exchangers, who have lower spreads to begin with. You're never going to see a *higher* spread on larger amounts though, so it doesn't hurt to try. I would just caution readers of the thread not to expect the kind of results you saw. A PP mentioned a few dollars on a $1000 transaction - that seems like a more reasonable expectation. I wouldn't expect more than a half percentage difference, at best.
Thanks for sharing - this isn't something that a lot of people would think about!