I use this mirror set up, but generally without over-the-shoulder checks. I can say that I've nearly hit people while changing lanes on multiple (but infrequent) occasions, when they were originally two lanes away from me, not visible in my mirrors, going slightly faster than me, and then changed lanes to the one beside me. They seem to sneak in during the second or two between when I first look in the mirror and begin the maneuver. It's a rare occurrence, but not rare enough for my comfort. I have my turn signal on the entire time (I start signalling when I intend to turn), and this still happens to me. I'm definitely going to start using over-the-shoulder checks again.

And that's why you should still do shoulder checks.