If you wanted to have a really crude estimate.
Look at the distance penalties for the corresponding spoiler failure cases on landing, and apply them. It's very crude, but if loss of the OB pair of spoilers causes a 10% distance penalty on landing, you might guess that having one of the pair stuck up as a spoileron might cause a 5% TOD impact.
(Yes, I know there are braking efficiency issues and different speeds and what-not; I did say 'crude')