I understand all those reasons you've all mentioned, but it just doesn't seem that hard of a thing to do. The effect you're going for is to hog them out slightly bigger than the exhaust port so that you get a scavenging effect, correct?
I guess one drawback is that the 3800 manifolds don't look like they have a lot of extra material. The BMW manifolds were thick, like the old Chevy V8'*. Just from eyeballing the 3800 manifolds, it looks like they are pretty thin to begin with.
I don't know if I'll ever try it, it just seems like it'* within the realm of 'do it yourself'.