Grimaldi wrote:
An inexpensive thing to try is buying a 2x telextender for use with your 70-200mm. At f/5.6, you'll have the same speed as the 100-500 but not quite the reach. Focusing would probably not be a problem with the new cameras. If it doesn't work for you, you won't be out much money.
The extender does not work on the 70 to 200 because of location of the first lens element. I assume the 2x will be similarly restricted.