If it is badly restricted it can rob much more power than that. I think that 10% might be on the outer edge most of the time, but far more than that is possible with extreme cases being more like 30% or even more.
I remember a Pinto that I worked on that almost would not run due to carbon and sludge built up in the intake, and I remember a Ranger 2.3 fuel injected that was also about the same. Both ran reasonably well once the intakes were cleaned up. The Ranger had to have the intake manifold removed and hot tanked it was so bad. It was so restriced that you could not insert your finger into the open ports.
No, I have no idea as to how the owners abused them to cause such an extreme condition.
