It is always best to see the stock outperform the commodity. Doesn't mean one can't catch a bottom but that is always a countertrend trade which is catching a falling knife idea. If you are early it will be costly. When the stock is outperforming the commodity the odds of success are higher historically as the trend is with you even if the commodity flattens out.
As far as comparing the Can listed stocks to the U.S. listed same stocks it is logical that the difference is currency as a rising Can$ will mean lower Can stock price and visa versa. But comparing the Can$ over long time periods does make one think there is something else as well causing such a discrepancy. I've never read any other reasons for this but looking at the Can$ that tends to chop sideways at par much of the time there seems to be more than currency that is causing a lower performance with Can listed stocks. I just don't know what else it could be.