I would bet that the gains from practicing the SATs are relatively high for people who have strong verbal abilities but are not proficient in English.

Also, since the SATs are optimized for U.S. students it might be the case that there are question types which U.S. students have seen thousands of times and so because of diminishing marginal returns would gain little by additional study, yet a Chinese student might never have encountered a similar problem and so would massively gain from practicing it.