I've purchased quite a few Showcases, and it doesn't seem like any of them led to sales. The only luck I had was on Cyber Monday, but I also had free shipping and Cyber Monday tags, so it is hard to determine what exactly led to the sales. I would get a small increase in views, but I've seen a better increase in views since I started being active in Teams, Treasuries, and as I've added more items to my shop. I don't buy them any more.