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Ukkisavosta - that is a great shot and the F2.8 absolutely could make a third party lens interesting. Can you go into detail about the lens, good things and bad things? I suppose a 2.8 300mm with an extender could be deadly.
Thanks a lot. I'd say the biggest issue with the 120-300 OS is the weight (2,9 kg) and size. In comparison, the 400mm f/5.6 is much smaller and weighs only a little over 1 kg. Apart from that, there's also the obvious question of getting a good copy, as there have been reports of QC problems with this lens related to focusing. The first batch of these lenses would not focus when pointed straight up, and I've read reports of focus varying between different focal lengths, but I believe the 5DIII's micro-adjust can compensate for this, if this should be an issue. Mine was good straight out of the box, and focuses consistently and accurately on my 60D.
The 120-300 OS takes TC's very well, and I've used it with my 2x Kenko Pro 300. With the 2x, focusing is not as fast, and the image quality in general takes a hit. I actually got a 1.4x Kenko Pro 300 a couple days ago, but have not had the chance to try it out with this lens yet.
Anyway, I'd suggest looking into the 120-300 OS as one option.