yes, it almost doubles the cell count. Because it keeps the yeast cells in solution better than if they settle out they can multiply more quickly. It also draws in more oxygen which helps growth. Consider using sanitized foil on the top instead of an airlock and swirl a few times a day and it should improve also
i use a conventional starter. i havent purchased a stirplate so i do mine multiple days in advance. Use mrmalty.com calculator to figure out pitching rates