The 100-400 f/4.0-5.6 L is a great lens. I had 75-300 and the 100-400 L glass is so much better--much,much sharper. You might be able to crop to get the same area of view as with an equivalent 600 with your 75-300 and the teleconverter and the same quality of picture or better.

It's a heavy lens, though. You just can't get light, cheap and good. In fact, it's hard to get light and good telephoto lenses.

If you buy the lens, you'll love it, with or without the extender. Unfortunately, once you start buying L lenses, you'll never go back. They are so much sharper.