During this Spring girls high school soccer season, I've used a 1.4x extender attached to my 300mm f/2.8 lens even under high school stadium lights. With yesterdays game here in Michigan being the first warm weather game allowing me to use two camera bodies comfortably, I attached a 1.7x extender to the 300mm lens and the 1.4x extender to my 70-200mm f/2.8 lens. More of the 300mm / 1.7x shots were soft compared to the same lens with the 1.4x extender attached. But the reach of the 300mm / 1.7x combination was very useful for soccer. And the 1.4x / 70-200mm lens combination is perfect for near-side soccer actions.
Nikon D700; 70-200mm f/2.8 with 1.4x extender; 1/3200; f/5.6