The use of a tele-converter is not as simple as slapping one on a lens and getting more magnification. There are several issues with tele-converters you need to be aware of.
1st. Tele-converters work best on top quality glass, and are usually manufactured to work with specific lenses. Canon and Nikon are two prime examples. Their TC's only work on particular pieces of glass. Several 3rd party manufactures make TC's that will slap on anything with a wide range of results. Even a top quality third party TC will produce poor results with less than top quality glass
2nd. Tele-converters work best on top quality prime lenses of at least 100mm or longer. That is what they were designed for. They can produce some good results with the proper top quality zooms over 100mm. The are not designed to say make a 50mm lens a 75 or 100 mm lens.
3rd. Tele-converters will cause a loss of light. A 1.4X,1.5X, or 1.7X TC will cause the loss of 1 full stop of light. A 2X TC will cause a 2 full stop light loss. That can mean poor to no AF function depending on the body you are shooting with and the lighting condition.
4th. Tele-converters magnify. Your thinking, "Well Duh, that's what they are for." You are correct they magnify. They magnify not only the focal length of the lens they magnify every little quirk, foible or short coming of the lens it is attached to. Put it on a consumer grade lens that is slightly soft in focus and it is going to magnify that softness.
Used properly a good quality TC can produce excellent results. I am quite happy with both my Canon EF 1.4X II and EF 2X II TC's on my 300mm f2.8, & 400mm f2.8. Most often however a cheap TC bought with little research or understanding will find a home in the bottom of a bag never to see the light of day after that first try or two.
If you want that Sports Illustrated close/clear photography quality, you are going to have to spend the kind of money that sports photographers spend. I know, I'm a sports photographer and I have spent it for top quality glass and bodies. I don't own a lens that is slower than f2.8 and that does not come cheaply. I rarely use my TC's for sports. They get pulled out when I am in the mountains of New Mexico shooting wildlife. When shooting real wildlife of the dangerous kind when a Grizzly fills you viewfinder and you are not using at least a 400mm lens we have a name for you. That name is DINNER!:mrgreen: That is when my 400 has my EF 2X II attached and my 44 magnum is ready in the holster.