Vishnu no sell catback, and I don't think M2 does either.
No idea about Stromung versus BPM. I've got a hybrid catback that's a Stromung midpipe with a Prodrive muffler. Sounds great...and my butt dyno is convinced there's a bit more torque...but it's probably not much. I'll be getting a downpipe soon I think. If you have the dough, I'd go with a full Vishnu Turboback...the thing is a piece of art.