Neil, did you use a tripod? And did you get the Nikon TC-20eIII?
I have been using my 2x TC with no problems whatsoever even wide open. Just pop over to my wildlife thread and see. Most of the pictures are shot with the 2XTC
One thing you need to realize is that with the 2x TC your focal length becomes 600mm. Which means your long lens technique will be put to the test. Any instability or vibration not taken care of will show up in your pictures. That means a proper head/gimbal, a strong tripod, good handling technique, as well as knowing your exposure variables.
This is my initial support setup. The tripod is not a small one (M-3204) and a Jobu Jr Gimbal head. Even with this setup, I was getting vibrations like no tomorrow when my mirror is flapping. it was so bad that the subject in my viewfinder looks as though it was bouncing up and down constantly. I missed many shots due to this, due to the AF point moving around due to my viewfinder bouncing all over.
Picture of the entire setup deployed.
Even though I was suffering from the vibrations, I was still able to get some good photos. And no, I did not have fill half the frame as bro Luminare suggested. The following picture is cropped too, due to the bird being very small and quite far away. This picture is shot with the setup shown above at that same very spot. It is doable actually.
Anyway this is my current setup, and even with the 2XTC, I can get great birding pictures. Note the size of the tripod, as well as the additional lockdown to prevent vibrations. No more vibrations like before, Zero. I think I went overboard, but then again, if and when I get a bigger/longer lens, I can use the same setup.
All I can say to you is, do not be too quick to discount the gear. Have some patience and learn how to get the results some folks are getting with the same gear. especially now the cam body you are using is newest and top of the line. It is not the gear but your technique. Work harder, learn and practice. Birding is not as easy as many think it is, my friend. Doesn't mean you get the gear means you can shoot the subjects well. A lot of technique involved too. Especially in sekinchan where you are on a boat, and light is not the best due to the shadows cast by the overhanging trees and using a TC brings your aperture down to F5.6, you really do need to watch your shutter speed.