i have tried the lens with most of the available tc's, (canons, sigmas. kenkos, promasters) i would say that i use it 50% 1.4x, 25% 2x and 25% bare. i think the canon 1.4xiii and 2xiii are good. i have had the new sigma tc-2001 for a while and think its a bit better than the canon (about the same sharpness but less ca and faster focusing) i just got the tc-1401. my initial impression is favorable. if you already have the canon series iii converters then use them. if you don't and are buying tc's specifically for this lens then get the new sigmas
adult eagle 70d canon 1.4xiii (420)
immature eagle 7dii sigma tc-2001 (600)
crane crop 7dii sigma 2001 (600)
crane in field 70d canon 2xiii (600)
ps. the sigma 2001 and the kenko pro 1.4x stack and look suprisingly good on the 6d, its 840 f/8 and still focuses pretty fast. its not the camera i use for long shooting but it might be a pretty useful setup on a 5diii?