I have only ever watched one cam recording of a movie before. I think it was 'The Illusionist.' I was at a friends house, he slapped it on. The color saturation was wrong, the sound was like mono and the camera appeared slightly off so the very top or bottom of the screen was cut off. It wasn't completely unwatchable but it spoiled the movie. Its like watching a film through a neighbours window.. crap.
Piracy has far more effect on DVD sales. This is for 4 reasons. The price of DVD's, the inconvenience of going out and getting the DVD, the bullshit adverts and unskipable junk before the film and the menu which takes 20 seconds to display before you can press PLAY.
If there was a DVD quality recording of the avengers I think it would have easily had 5x more downloads, probably 10x. However, realistically the cinema is an experience. If you enjoy the cinema you are going to watch a film like this at the cinema - then maybe download it. I am firmly of the opinion that if you make a blockbuster film, whether it leaks or not it will do well.