It lasts just fine (Source: I've done it)

The M3 has knock sensors, like everyone else.

https://www.google.com/search?q=bmw+m3+knock+sensor

If you look up engine knock on wikipedia, it will tell you the same

From: https://en.wikipedia.org/wiki/Engine_knocking#Knock_detectio...

"Due to the large variation in fuel quality, a large number of engines now contain mechanisms to detect knocking and adjust timing or boost pressure accordingly in order to offer improved performance on high octane fuels while reducing the risk of engine damage caused by knock while running on low octane fuels. "

Just about every modern car comes with knock sensors and computer controlled knock prevention mechanisms that will compensate for lower octane, especially on high strung, direct injected, twin-turbo cars like a BMW M3. The only real downside is increased emissions for using non-optimal fuel and lower engine efficiency/power.

It'll work just fine, it will however not run at full power.

It will last at least as long as the warranty.

Cars in the US are required to be able to run on 87.

Look up engine knock sensor.