Like you I am aiming for a minimum of 1/1000th. This is easy now with the 5D MK III, happy to go to 12,600 ISO. Thats why I was thinking the 2x Extender would be OK at F5.6 with a high ISO, but maybe as others have said the 1.4 is fine - although with the 2x I could get some good moon shots as well!!
I would need an ISO of 12,800 to use my 100-400L in our OHL arena, too. I've only had the 5D3 a short while, and have tried that ISO just once during a visit to the zoo. Results were not too bad with a little NR. I may give that lens a try, too, next hockey season.
I wouldn't even consider the 100-400L with the 7D. I was pushing it with the 70-200 f/4 on that body.