6D AF at f/8 masquerading as f/5.6

After I noticed how accurate the 6D's center AF point is, even in low light, I tried a couple of f/4 lenses (300/4L IS and 70-200/4 IS) with two Kenko Pro 1.4x TCs stacked, since only one would be recognized.  In a room lit at night by a single compact florescent light, both lenses allowed accurate AF without any hunting, except when contrast was lacking or too fine (shooting a SKU code from across the room, the lens would overshoot it, by it focused fine on my small fake bird and cat, and the lettering on many boxes.  Anyone else try any other combos to fake f/5.6 and had success?

BTW, every lens I've tried is close to perfect in AF accuracy, without any micro-adjustment.

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John
http://www.pbase.com/image/55384958.jpg