6 oz in a small container on a porch (is your porch covered?) is not a bad yield. If 20 gallon is too big/heavy, try a 10 or 12 gallon pot. I don't think altitude makes a difference in yield; if you have a seed strain for cooler climates (like those used in B.C.) that is used to shorter growing seasons might improve future yields as well. I don't know if cool temps will effect your little lady until they reach near the freezing point.