I agree with the statements here, and i can sympathise. When i was using a 70's Tamron 70-350, i though it would easily do the job, and of course it does to a certain extent, but not for small or medium birds.
It was fab for big birds, and animals but i went to a 150-500 for the smaller stuff. Bushcraft is good, if you have the patience and time. Not everyone has that luxury, so the only way to get closer is a longer FL.
I've seen some great results with the Kenko DG 300 Pro 1.4x, which will give your Tamron 420mm at the loud end. It will cost you 1 stop of light, and barely any IQ degradation. A 2x will be slightly worse, and losing 2 stops. I've seen a shop that sells the Kenko for new for £97. A bargain really.
I'll be grabbing one at some point for my 15-500
Just to give you an idea, here's a shot with my old 70's Tamron 70-350 / constant f4 but with a Tamron 2x TC giving 700mm @ f8 Max
http://photo-sharing.winsoftmagic.com/1/1b8i7dcbqe.jpg
Both the lens and the 2x TC are old and no longer made. The kenko quality will be even better with a more modern lens