What can I say, I live dangerously. That $3 bought me a hot dog and a coke for lunch on Sunday. But honestly I would love to see at least one M42 that died because of low octane abuse on this forum. This one has been on a steady diet of 87 octane for the last 139k miles. Why would I do anything different? Come to think of it, why would anyone offer aftermarket chips to take advantage of high octane gasoline if 87 will kill this engine? I'll let you know when I kill mine.
I don't have time to argue about every one of your points but I have a nice little before and after the diff swap comparison test lined up. You crack me up 
Ill be interested to hear every last tidbit of info on the diff swap - diffs have been the bane of my existnce in otherwise glorious ownership of this car. Noisy, noisy, noisy from the diff itself - forget the engine...
Anyway, provided the radio isnt too loud, it would be VERY evident if 87 was a real prolem for your car. I did an experiment with 89 in my 318i (chipped, though, so this can be a major difference), and while around town it was OK, more than half throttle on, say, highway acceleration on a merge would make a notable knocking sound.
If you were having problems, youd likely hear it, its not that subtle, even when quite quiet.
JMH
