I've had my 70-300 about 5 days, and im very impressed (enough to sell my 55-200).
Used it with both converters at 300mm, and i found the 2x very difficult to hold steady, the 1.4x is a lot easier, but even the IS and OIS miss a few shots.
There is definitely a minimal loss in quality with both converters, as to be expected.
I think using it with the 1.4x is a great option to have, but the 100-400 would be a better.