How dark of a roux do you make? I know the thickening power reduces the darker you make your roux...
I made a lot of gumbo last week and used about 6.5 cups each of flour and oil. I make my roux very dark, pretty close to the color of dark chocolate. It darkens a little more when I add the vegetables to it to simmer. I don't like light rouxs in gumbo, though, so this is my preference.
I also don't make the slow roux. I use PP's method and it probably took me 30 minutes, tops, to make that roux as dark as I wanted it.
I don't understand what you mean by "thickening power" reducing "the darker you make your roux".
Mine is the same color whether I'm making 2 gallons or 8 gallons of gumbo. You just have to stir constantly and be patient. When it hits the mark, remove from the heat and add the veggies. Stir so it doesn't stick and then simmer the veggies for a bit, until wilted.