I used to go up to F/L 370 occasionally, and F/L 390 once, in the A300B4-200 after carefully checking the buffet boundaries.
The trouble was that the company Ops Manual did not specify anything relevant to buffet boundaries, and a few of our "Aces" went as high as possible in the belief that as high as possible must be better.
After once sitting through 2 hours of low speed buffet whilst paxing, I jotted some shakily written performance numbers on the back of an envelope and handed it to the boss upon arrival. I was alarmed when my hastily prepared (and unchecked) figures were incorporated into the Ops Manual, but the buffeting events ceased forthwith.
I think that that was the day that my obsession with EAS began.