With the teleconverter this would make the lens a 500mm and what f stop?
Is this probably the next best upgrade from the 100-400 canon EF IS USM lens?
I am not above using Sigma if their is a comparable sigma? It seems like sigma either has OS and regular speed auto focus, or HSM and no IS though in their lenses.
Thanks.
It would make it a 600mm (2 x 300... 
) f5.6 (2 stops because of the 2x).
Is it the best upgrade? Depends what you want. A hand-holdable lens with faster AF and more reach...then maybe yes. Best quality at 500mm or greater on a tripod? Probably not.
Sigma do a 300/2.8 but it does not have IS - so maybe not so useful for handholding.
I've thought about the 300 f/2.8 and TCs (either a 2x, or maybe stacked 1.4s) as my next upgrade for quite a while, but I'm starting to entertain the 400mm DO with a single 1.4 instead.
It's much lighter than the 300, still gets me to 560mm, and - because xxD bodies will work with either of these lenses and TCs without taping pins down to f/5.6 - will provide faster and more reliable AF.
They're both a hill of money though!