When you use a 2X converter on a F2.8 lens you loose two stops and the widest opening you can get with this combination IS F5.6. On my Nikon 70-200VR F2.8 when I am using my Nikon 2X teleconverter I shoot at F 5.6 just about all of the time with great results.
Pairing the lens (Nikon 300 mm 2.8 vr II) with the latest TC-20E III teleconverter produces a compact and usable 600mm solution. The focus ability and sharpness do degrade however if stopped down to around f/8 and combined with good lighting conditions, usable results can still be achieved.
F / 8 aperture is too closed for me. So is a better solution 1.7 teleconverter???