It's about 3 lbs per corner. And definitely noticable in the handling of the car!
The 365mm rotors will of course weigh a little bit less vs the 380mm, but not much.
If your wheels clear the 380mm kit, I would get that one in the front (if you track the car regularly - otherwise it's $$$ overkill for a street car).
I have 355/345 mm on my track-prepped 335i and it works great.
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