Adding a 1.4TC would make it, at the long end, a 560mm f8 wide open. However its probably more like around 540mm f8, as its not a true 400mm, but more like a 385mm. Not that its a big difference.The latest 80-400 is a cracking lens but look at the price,especially if you add the converter.
Hi,
At this point, what I've read so far, I would go for the Sigma 150-600 Contemporary.
It seems to be sharper as the Tammy and with the USB dock (accessoire), you can adjust much more.
Ad B