If anyone is wondering about the 300 f4 and it's performance with teleconverters I must say I am continuously impressed with how sharp it is. I have even found the AF to be sufficiently fast with moving subjects. This was with the 300 and 1.7 on a D700 - Shot wide open at f6.7 (widest aperature when paired with the 1.7 TC) 1/1500 of a sec in AF-C mode. My dog was running straight at me and while not all the shots where this sharp, they were all very useable.