SeriesID doesn't affect that, it's behind-the-scenes and affects how the books group together on the Series page. Is possible to have books with same Series but different SeriesIDs. They'll show up as separate on the Kobo.
EDIT: Two recent acquisitions give an opportunity to demonstrate the accursed Pocket Books issue - 40 books I already set to the same SeriesID with the
SQLite query, so they're a single series.
But the two other books, which I haven't merged in yet, have it set to different values (GUIDs provided by the Kobo store). So they show up as two separate series of 1 book each, even with the same name.