I have been following these issues pretty closely over the years I've been on the various MS forums, but I will go through that thread some time and tabulate the number of smoking exhausts correlated with mid-pipe cat deletes. So far you have pointed to one, but that's gotta be counted as anecdotal even though it's your buddy. And what kind of smoke? Billowing smoke? A little smoke at idle? Smoke on hard acceleration? If one is meticulous, a little smoke could be taken as something more serious than it is. And only after a few weeks? That's pretty soon if the idea is that it happens eventually and eventually will happen to everyone with an exhaust mod. Did he get a new turbo b/c of the smoking? Also, there have been smoking turbos on completely stock cars and those with just the MS-CAI, so that in itself should be enough to show that it isn't caused simply by any and all exhaust mods.
One thing I'm pretty sure of is that there will be zero smoking exhausts correlated with an aftermarket CBE. The stock CBE has been shown to be very free-flowing already. And it seems unlikely that Mazda would offer an MS option that risked causing turbo seals problems b/c of the reduced back pressure. Not out of the question, but unlikely.
DP's may be another story, especially catless DP's. But even there, it may that the cats are just masking smoke that is there with or without exhaust mods. It seems more likely to me that the substantially reduced backpressure may simply allows turbo to spin out of it's efficiency range.
Lastly, I fail to see how the whole forum is backing you up on your simplistic single-factor analysis.