That is not a specific slight at you but more a response to the post which seemed to suggest that the kit should be producing good images - which of course it does with care. We are all guilty of shooting at the edges of capability at times - and have to accept that in those situations it may well be the human part which is limiting.
As for the TC 2.0 III - mine is fine also, some loss but really not that bad and massively improved from earlier 2.0x models in both Canon and Nikon. You may have a duff copy, in which case get it replaced. I wouldn't rule it out of court though. In certain circumstances it can be the difference between getting an image or not - which is why it has a place in my bag. You do though as has been covered have to treat it like a 300mmx2=600mm focal length and get yourself well above the 1/focal length shutter speed and at that length unless you are experienced and strong probably 1/2xfocal length even with VR engaged.