Petr Zavadsky (CZ) , Jun 09, 2002; 06:28 a.m.
2x doesn't work well with anything unless you stop down 1-2/f and event than it is compromise.
Also Dan Heller using this lens almost exclusively use it with 1.4/2x TC's.
400/2.8 is almost as heavy as 600/4 and almost the same price. Go with 600/4 if birds are your main subject, wide open it will be much better than 400/2.8 with TC wide open. Leave 400/2.8 to sport photographers :)
My choice of lens would 500/4 due to weight/price/convenience of use issues, but beeing pro, I would rather take 600/4.