I've been reading up on the Sigma 70-200 f/2.8 because I don't think I'll be able to afford the Nikon version, either the 70-200 f/2.8 VR or the 80-200 f/2.8 AF-S ($1700 and $1100 respectively vs about $850 or so). There is an 80-200 f/2.8 AF, but the focusing isn't as fast, thus its a few hundred less than the AF-S version.
I've seen a few reviews on the Sigma 70-200 f/2.8 recently in some of the UK mag's. They all give it very positive reviews. I don't know for sure if the 70-200 f/2.8 VR is $900 better than the Sigma. Granted the Nikon 70-200 f/2.8 VR is one of THE best Nikon lenses and anyone who shoots Nikon would love to have one, but it also is $1700.
As for the teleconverters. The Sigma's are good, especially with a Sigma lens. No matter which 1.4TC you go with you will lose 1 stop, so with it on the f/2.8 actually becomes f/4. Which is still pretty good for that range especially on a digital body with the 1.5x crop factor. I think for this lens range a 1.4TC is a good thing to have, definately cheeper than getting the 200-400 f/4 VR (which runs about $5K).
As for the VR with the 1.4TC, I know I've read about the combo together, but I don't recall what the vertict is (does the VR still works effectivly or not, I think it does, but I don't want to say for sure) so be sure to look into that before buying. You can probably do a quick search on nikonians.org under the Nikkor Lenses forum. I think that is where I've seen it discussed.
Happy shopping.