@trytofly:
I can't give you the figures of the -500, but the principle, I guess, is the same as for the -300/400:
There is something called HSBL (High Speed Buffet Limit); at altitudes above 25.000ft you see a yellow bar beginning to extend downwards from the Vmo "barber-pole" on the airspeed tape, providing a .3g margin to HSB. Since this effect is related to the Weight - ambient pressure ratio relation, HSB is most prominent at high altitudes and high weights.
Flying into the HSB results in the wings starting to vibrate which is not very comfortable (spilling coffee etc...).