I have had a look at Efficient's sectional times and I am not keen on him for the Cup.
Firstly, the time they ran in the Derby was very slow, at least relative to the Saab. 2:38.35 (Derby) v 2:35.86 (Saab) Give or take, a second = 6 or 7 lengths. So that's about 15 or 20 lengths quicker in the Saab than the Derby. Plus the track was drying out little by little throughout the day.
Maybe Better ran home his last 600m in 35.97, off a considerably faster pace, under 57. This was a most impressive performance. Efficient ran his last 600m in 35.22, also impressive, but the reality is that any half decent 3YO should be able to run that sort of time off a farcically slow pace. The performances by the leaders were ordinary - they are an awful bunch of horses, and Efficient had every right to run them down, especially with the ride he had.
Efficient clearly has class, but whether he can stay 3200m I don't know, and it isn't really possible to say off this run, because he didn't have any kind of staying test. Anyone who tells you otherwise doesn't know what they are talking about or is a liar. They walked, and he outsprinted them. However, bearing in mind that he is not especially well weighted, is only very lightly raced, unproven in a genuine staying race, and is, of course, only 3YO, I would be prepared to bet against him.