Hi "JC", if I were you I'd get myself a household CO alarm for peace of mind re cabin fumes. I only very rarely actually smelled any fumes (with an ill-fitting clamp) so your issue seems quite a bit more serious if you're getting regular stink-outs. My CO alarm did actually go off once - thankfully only while I was in a large retailer's underground garage full of everyone else's very smelly car fumes. So with my issue, the clearest symptom was the engine-bay soot.
To compare, I'd photograph the engine bay and bonnet underside, clean the sooty parts and check again after a few days.
If the soot trails are similar to the photos on this thread, then the clamp replacement might be a good bet.
At least it's not anywhere near as expensive as cat and/or particle filter replacement.
Thing is though, I didn't have any power issues to speak of- so your power drop-off seems to point to a different issue.
If you're on a warranty, I'd want to push for a restoration of normal power delivery, no matter what the specific issue is, and let the BMW techs work out what needs doing. Have you asked for a spin in a same-engined demo car, together with a mechanic, and then swap back afterwards to your car to get a concensus about the lumpy power on yours?
The CO alarm would also help your case if it was being set off regularly in the cabin...