Nothing short of a several thousand dollar supertele will get you there while maintaining f/2.8. But, the reduced ISO noise with FF should make up for that. Putting a 1.4x TC on your 70-200/2.8 II would get most of your lost 'reach' back, and the IQ hit isn't too bad with the 70-200 II (I find even the 2x produces usable results, rare for a zoom). Althernatively, consider the 70-300L - smaller than your 70-200/2.8, if you can live with two stops higher ISO for f/5.6 you get 300mm and excellent IQ, along with great IS.
Exactly. Remember, you can also crop fairly significantly from the 5D3