In my search for materials, I came across SOAS in London. They offer courses in Korean as well as TOPIK prep-courses.
They use the Sogang books, and I figured they would know how the Sogang levels correspond to the TOPIK levels.
I therefore sent them an e-mail to ask for their advice and they were kind enough to answer my questions.
The following is information provided by the courtesy of Ms. Jaehee Cho, language specialist at SOAS language centre. Please note that the following is based on her teaching experience, not official guidelines from Sogang University:
TOPIK level Sogang Korean
Beginners level 1 – SK 1A,1B
level 2 – SK 2A (2B for a safe side)
Intermediate level 3 – SK 3A,3B
level 4 – SK 4A, 4B
Advanced level 5 – 5A and/or 5B
level 6 – 5B and other newspaper articles or literary texts.
I hope that helps you plan as much as it has helped me 🙂