Hongkongfooey,
We came though SIERA on Saturday and explicitly asked ATC whether 280 at SIERA meant exactly 280 or maximum 280.
They replied it's maximum 280.
Also it is a "Speed Limit Point", ie there is a limit there, a maximum or a minimum value, as opposed to an exact requirement.
I need to ask a few more ATC blokes but I think SIERA has just been been rolled in with all the other inbound point speed limits with no real consideration given to the SIERA arrival's intricacies, espcecially if you are cut in. I'm guessing we will see a change soon to "Max 280 kts"
So I flew my normal 250/FL190 at SIERA in anticipation of track shortening. ATC had no problem with that. Still ended up being given the new 5C due to the guy in front of us slowing down early.