SB is essentially a division within a ministry that has gotten so big, they are branched out to have autonomy on its own. To me personally, the establishment of SB is merely to "feed" those scholars and ensure they have a job to do, yet making it seem important. Granted over the years, there are some SBs that have truly define themselves and make themselves indispensable.

You cannot say tier 2 SB = CS in terms of remuneration. Each SB have their own way to peg their remuneration; some are more generous than CS, some are way below CS. Rather than getting fixated on whether they're tier 1 or 2, it's more important to just compare their remuneration, as well as career development/progression. Culture is another important factor. As a rule of thumb, the better the remuneration, the busier and more hectic your job will be.