I think another factor that should be considered in research like this: the second viewing.

If a movie is good, the story is good, etc., a second viewing will always be worthwhile whether you've been told the spoiler, or have actually seen everything that's to come.
The question should be, in what way does the first viewing with spoilers, differ from a second viewing after seeing it without spoilers?

Sure, in a second viewing, attention will be paid to the elements and aspects you've already seen, rather than an abstract concept you've been told about (or even a scene shown from the ending) consciously anticipated.
It's a major difference.
Then, you can be told different things in advance, from the final twist, to the entire synopsis.

Still, again, when I was viewing the 6th Sense, I couldn't shake off the feeling that I'm kinda viewing it a second time.
I already knew what it was coming to, and what the little deceptive elements were there to do, and I feel I'd been watching it about the same way had it been a second viewing.

So that's another important factor ;)

Then, with Lost or Shutter Island, I knew the twists, but hadn't read the synopsis of either, AND, the way the illusion was staged was more convincing without the "dead-giveaways" like the wife not responding to Bruce Willis, so... there was a cognitive dissonance involved there. "I know this is how it's gonna turn out, but I can't imagine how, this is just not where it's going... not it must be flash-sideways, the spoilers must've been wrong".

And there yet another example how knowing the spoiler can have different impacts on enjoyment depending on structure and implementation :)