I know there will be slashing of a few scenes in order to account for time constraints, censor boards, and creative differences. But theoretically if the change were to be represented as a percentage, how much different will the screenplay be from the final product?
Professionally written screenplays don't get slashed. Rather, can't get chopped. Every scene and every beat is crafted with an objective. Most of the time the content get pruned becasue during the exercise of principal cinematography, the length unreasonably gets added for many reasons. That won't or shouldn't construed as the fault of the screenplay but the unscrupulous conditions or designs of production or direction. That's why the presence of screenwriter in a group of congenial crew members can make a lot of positive solutions to such problems.