I think that you would lose a bit of image quality with the TC on the D7000
The 1.3 crop mode with change the crop factor to 1.95 (1.5 x 1.3) from 1.5, so you gain focal length at the expense of reducing MP from 24MP to 16MP. Using a teleconverter with give extra length at the expense of losing light, image quality and focus speed. The loss of each of these depends on the TC used and the lens its used with.
A quick calculation would give the following with a 300mm f4 lens:
D7100 in 1.3 crop mode : 300mm lens at f4 x1.95 = 585mm equivalent 35mm focal length at f4 at 16MP
D7100 using 1.4 tc: 300mm at f4 x1.4 tc x1.5 = 630mm equivalent 35mm focal length at f5.6 at 24MP
D7000 using 1.4 tc: 300mm at f4 x1.4 tc x1.5 = 630mm equivalent 35mm focal length at f5.6 at 16MP
For me I see the cropped mode on the d7100 as the nearly the equivalent of using a 1.4 tc on the d7000 without losing one stop of light and being able to turn the mode on and off without having to remove the lens. I'm not sure if image quality will be affected apart from losing MP as I've never used a camera with it.
What I do find interesting if I've got my sums correct will be using a d7100 with 300mm f4 lens with 1.4 tc in 1.3 cop mode with give 820mm f5.6 at 16MP. This would be especially nice if the image quality is not badly affected, could be a cheaper way (and only way for me) of getting 800mm f5.6 than using the new £15K nikon lens
. A 500mm sigma would give a crazy 985mm 35mm equivalent in crop mode and hopefully without any loss in quality or light.
The questions I want to ask Nikon at Focus if i can are how well with the d7100 work with the 300m f4 and 1.4 tc in crop mode, if image quality is affected by the crop mode and if the crop mode is better than cropping in post processing.
I'm happy with the 16MP sensor on the d7000, there is more than enough pixels for me. If I could have the benefit of having the equivalent of a 1.4 tc that I can turn on and off at a push of a button, without losing a stop of light or affect auto focus speed and still have a similar amount of MP to the d7000 that sounds great to me. The more I think about the d7100 the more I talk myself into it, I'm thinking of getting a 2nd body and this could be perfect for me. Ive been really happy with the d7000 and I don't really need the build of a d300 so I've no need to wait for a "D400". I'm waiting to see what the first people who buy it make of it and see how the reviews go.
