The amount of improvement varies widely, depending on starting rate and comprehension. If a person is unable to finish the reading sections of standardized tests in the alloted time, the amount of improvement would vary with what proportion of the section was rushed and/or incomplete. If a person is also unable to complete other verbal sections and/or science sections on time, the score would improve still more.
When I work with students on test prep, I recommend speed-reading, for all slow or average readers, prior to test prep.Typical speed-reading results for slow or average readers are a tripling of rate with equivalent or better comprehension.