As I understand it the Nikon 300/4 is a cheaper lens than the Canon 400/5.6 so what makes you so sure that with a converter it will be faster focussing and better IQ than the bare 400/5.6?
As far as image quality goes I do know that the bare Canon 400/5.6 is right up there with the very best Canon superteles costing £5k or more(albeit it is slow at f5.6 and lacking I.S) So on this assumption you would rate the Nikon 300/4 probably better than any Canon lens.
I would personally be very surprised if the Nikon 300/4 + tc was faster focusing than the bare 400/5.6 - if it was then that would surely make the 300/4 easily the fastest focussing lens around (of any make) and I have never read this about this particular lens. I am not sure how you would measure it though as the bare Canon 400/5.6 is near instantaneous for AF speed, especially if you use a focus limiter, it is know the be one of the very fastest Canon lenses as far as AF speed goes.
I would also doubt very much if the image quality would be so good either. Like you this is pure speculation on my part but common sense says that teleconverter's both slows AF and also reduces image quality to some degree.
If you were to reverse the comparison whereby a Canon + tc was being compared against a bare Nikon lens, then I would have no hesitation in assuming that the Nikon would be faster to AF - its just common sense.
Roy
Let's don't get into a discussion of speculation. It's off target from the intent of the OP. Since you have not used the Nikon 300 f/4 with or without the teleconverter, it's really not a reasonable conversation. I did use the Canon 400 f/5.6 yesterday so I can speak with some limited experience, but I don't own it.
The person I was with did have praises and some minor criticisms of the lens - it's older design and AF speed. I share those thoughts for birds in flight but could not tell you whether it was the camera, the settings, or my experience with the gear. The Nikon 300 f/4 is a fast AF lens in the scheme of things. How fast is splitting hairs as the f/2.8 long lenses from Canon and Nikon are likely a little faster. The 300 f/4 is an internal focus AFS lens so it has an internal motor that helps assist with fast AF regardless of whether or not a TC is used.
Your reference to the price of the two lenses is not correct in the US - but certainly may be the case in the UK. In the US the 300 f/4 is $200 more expensive than the Canon 400 f/5.6. Again - that is splitting hairs and is not on point for this thread.
Image quality of the Nikon 300 f/4 compares favorably to the Nikon 200-400 which is a first class zoom. It also is very close to the Nikon 300 f/2.8. When you put a teleconverter on the lens you do get a slight drop off in quality but with the Nikon 300 f/4 the drop off is much less than normal. The 1.4 teleconverter has no noticeable drop off. The 300 f/4 with 1.7 teleconverter has a minor dropoff in image quality but it is still good enough to use for critical images.
I think the point for the OP is that the D7000 is a fine camera. The Nikon 300 f/4 AFS lens is a good solution and the lens is one of the best values in the Nikon lineup. The 300 f/4 works well with a teleconverter - especially with the 1.4 teleconverter for an effective 420mm f/5.6. And the one caveat - the 300 f/4 is likely due for an upgrade to add VR (and that has been speculated for 4 years).
My take is the OP should feel good about using the Nikon combination. DXOmark.com rates the D7000 sensor higher than the sensor on the 7D. There is more to it than the sensor only. The D7000 has very good battery life compared to most models. The AF system and processor are new, presenting some challenges when moving from other models, but the enhancements are viewed favorably once you understand how to optimize settings.
http://www.dxomark.com/index.php/Ca...brand)/Nikon/(appareil2)/619|0/(brand2)/Canon