"At the end of kindergarten and first grade, students in structured English immersion had significantly better English reading and language skills than students in transitional bilingual education. The WWC interprets these effects as corresponding roughly to the skill difference between the 50th and 66th percentiles of English reading and language achievement".
Slavin and his colleagues actually published the study in Education Policy Analysis Archives in 2011. Here is how they interpreted the outcomes (from the abstract):
On language and literacy indicators, "first graders in TBE performed significantly better in Spanish and worse in English than did their SEI counterparts. Differences diminished in second and third grades, and by fourth grade, when all students in TBE had transitioned to English-only instruction, there were no significant differences on English reading measures. These findings suggest that Spanish-dominant students learn to read in English equally well in TBE and SEI and that policy should therefore focus on the quality of instruction rather than on the language of instruction for English-language learners".