Just came back from Fotocasion, a big photo store in Madrid. They finally had the Canon Extender 1.4 mark III in stock, I have been waiting for this to happen since late December now. As usual I tried it out in the store, seemed all good. Obviously I will run some more tests, but again nothing chartish, pure real life functionality and purpose.
Thanks to this I am all set on the tele end for now, 450 mm will be enough for future safaris :). The ultra wide end will be fixed next weekend with the Canon 16-35L 2.8 (well short of a full frame camera that is)!