like what rhino123 mentioned, all t-cons will degrade your photo, reduce focusing speed, stop down your aperture.
if you need the range, yes it's worth it. Unless you are willingly to spend on the 400mm 2.8
or you can consider changing you 70-200VR2 to the 80-400VR, so you don't need a t-con, but, i'm not sure if the 80-400 will be sharper than a 70-200VR2+2x Nikon t-con.