200-400 f4 plus teleconverters.

I have had a Sigma 150-500 for some time but have been saving for some time to get something far better. I use a D300S, a gimbal head, strong tipod and am interested mainly in birds and wildlife. I have the 70-200 f2.8 and the 300mm f4. The 200-400 f4 seems the next logical progression but I can't run to the cost of the 500 or 600 lenses.

Now I have read masses of on-line reviews and am aware of the long distance sharpness 'drop-off' but would reall like to hear from users about the lens's 'real world' performance with all three tele converters. I have the 2tc mk 111 and 1.7 and will purchase the 1.4.

My 300 f4 is not supposed to work with the 1.7 coverter but on a bright day it works fine for me with little loss in IQ so could I just about get away with them on the 200-400?

Thanks