I find the opposite. Last month, for example, I finished D7, having started with 8 bonus stars, implying I gained 7 ranks (22 stars) without the aid of bonus stars. The result? Eight bonus stars!
So know not only do I have have to play 30 or so games against people of my own mmr just to get back to P5 (when my bonus disappears), but then I have a 22 star hole relative to last month. In the old system I only fell back 20 stars, consistently every month.
PS: My win rate in Diamond last month was 65%. I am time capped, not skill capped, at that rank.