It isn't about formality, it's about appropriateness.
You wouldn't wear a tux to a interview, would you?
A SB peak lapel is a bit flashy, not conservative.
I will say "probably not," because I suspect that they wouldn't meet Manton's standards. I also suspect you could probably get away with them in a lot of places that are a notch below pure-D CBD.
Thread title. Also, it comes up fairly often, at least in some of the threads I frequent.