Well, I am not sure if you can find an objective study...
This one seems almost reasonable:
White paper study (by a monitor company)
at least it lists some examples.
Honestly, I am not sure if a sure-fire study exists - it just happens to depend too much on the employee and how well they are adapting to a new situation. And how much they can increase their productivity by multitasking.
I honestly don't know a lot of people who can use 1280x1024 resolution well, or even know how much they could profit from pivot applications.
Ever since I bought an LCD, I program in portrait, not landscape mode, and I love it.
But the productivity increase would be hard to measure... Maybe I just liked it better, and was less tired when done, but nothing a company can profit on. Or maybe I was 5% fast in writing lines and finding bugs.
I am not sure that if there were a study, I would believe it.