Excellent info
I have some regular 2x TC's here ( no Barlows any more though ) basically a Panagor and a Vivitar, both PK mount, fully manual without contacts. By comparison, the Panagor is deeper in size from front to back and weighs significantly more then the Viv.
I plan to keep the Panagor as is, cos it just seems better built, but i may chuck a teleneg into the cheaper looking / feeling Viv. It was dead cheap, (as was the Panagor), so no worries about butchering.
I have a Tokina SD 70-210 on the way, so we'll see whats what.
Also, which may surpise you, i took a shot with the 120/600 today, bit hurried but the IQ is so much better than what i was getting previously with the 80ED at the beginning ( remember that ? ) , but i think this was down to the Canon 450D. I havent a clue why, but although i got round the problems in the end with careful PP work, i wasnt really happy.
There is a hint of CA, just like you said, but i knew this already, as i research everything i buy until i'm almost falling asleep
So, this good IQ could well be down to moving to the Pentax K200 body