I know it's tempting to go for an impressive 200 - 800 mm focal length setup, but if it didn't have its drawbacks everyone would own this combination (and Canon wouldn't be able to sell the new 200 - 400 1.4X for $10,000+).
Although they increase your focal length, extenders also 1) cut out the amount of light, 2) compromise image quality. The first point has the added complication that it can affect autofocus. All cameras have a minimum aperture size that it needs for autofocus to work, most are around f5.6, some high-end cameras go up to f8. The 2X will move up the largest aperture size of the lens two stops. So the 100 – 400 f4 – 5.6 will become a f8 – f11. Even high end cameras will probably struggle once zoomed much past 100mm.
You’re most likely not going to have auto-focus, so how do you feel about using manual focus only? The 2X will also compromise sharpness a bit, so you’ll probably have to stop down the lens quite a bit to get a result you like (plus the extender cuts out 2 stops). It’s fine if you have enough light, but it’s going to cut into your flexibility.
