Cmon Neuro, you know that one gotcha moment on a calculation doesn't negate the guy's whole knowledge and experience...
To keep this educational for readers, the f/stops represent a doubling of light at each step, and run in the order of f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8.0, f/11, f/16
Each step increases x1.4

Teleconverters increase effective focal length, but sacrifice light, as there's no such thing as a free lunch. They also add additional lens elements in the light path, which can soften the image.
A 2x teleconverter drops the light down by 2 stops, so you count two up the scale as Neuro mentioned in his correction. A 1.4x teleconverter drops the light down by 1 stop.