Research Software Engineers (RSEs) have emerged as a critical professional role bridging software engineering and scientific research. Despite rapid growth in RSE positions globally, their impact on scientific software quality and productivity remains understudied. We are conducting a mixed-methods study combining qualitative interviews with quantitative analysis of software repositories to understand how RSEs influence scientific software development. Our study examines RSE processes at research universities to understand collaboration challenges, impacts on software quality and sustainability, adoption of best practices, and effects on scientific productivity and knowledge transfer between RSEs and scientists. We present preliminary findings from this study, including both qualitative and quantitative data.