If I din't already own the 80-400VR and was in the market for such a lens, I think I would be tempted to look at the 3 Sigma offerings as described....they are largely all comparable. Bear in mind that 500mm is only 20% longer than 400mm. When you look at it through the viewfinder it seems barely significant.
The only frustration I have with the 80-400VR is its relatively slow mechanical AF which prevents photography of fast moving subjects. Not a lens for airshows or birds in flight. Equestrian events...it manages OK. Static subjects, pretty good. You have to spend about 3 times the money to get anything significantly better.
Some are saying the new 70-200 f2.8 VRII and TC2.0EIII give better optical performance than the 80-400VR and obviously also the faster AF-S and the option of f2.8 up to 200mm when you need it.
The older 70-200 f2.8 VRI and TC2.0EII combination was almost as good (optically) when I tested such a combo a long time back. It had better AF though.
So. Lots of options in a price range that veries from about half to twice as much as the cost of 80-400VR.
Interestingly - in the USA a rebate of $350 has just been declared on the 80-400VR.