Hmm Mr PC, we do indeed have a different opinion
My understanding (and indeed that of all my ATC colleagues - I hope, and pretty much every pilot I've spoken to) is that if you are flying a SID (for example) that is your initial clearance issued by the TWR controller and save a whole host of time rather than saying "after departure intercept the XYZ radial and at 2 DME turn left to point B crossing B at 4000' or above, speed restriction 250kts or less below FL100 etc etc". It saves a lot of time for all to say DVR 2M!
Now the SIDs and STARs are very carefully planned to allow for noise pref routings, keep a/c inside controlled airspace and indeed keep you well separated from all other SIDs and STARs within the airspace. They also provide for a clearance limit for the purposes of radio fail procedures and so forth. All of which, I'm sure, is no news to you guys.
Well once you are in contact with ATC in the air, any amendment to the issued clearance (the SID/STAR) removes the published restriction unless it is reiterated in the clearance and that includes speed. For example the STAR SLP may be 250kts by point X but if you are the only a/c in the sky or indeed if the controller wants you to be no. 1 of a series then they can cancel this restriction or indeed instruct you to maintain a higher speed until advised. It is then incumbent on the controller to ensure this information is passed to the next controller so they can allow for this 'non-standard' speed to be born in mind further down the line. As was previously mentioned this is within the class A airspace of a TMA type environment rather than 'The Bundhu of class G'.
The same applies for SIDs. The controller can cancel the published speed limit by saying 'no ATC speed restriction' and then the pilot has THE OPTION of increasing speeds if they wish. After all, the SID is simply a concise version of a verbal clearance that would have to be issued by the TWR bod. The next controller amends the clearance and unless reiterated, it cancels the previous.
That is my understanding and opinion although I'd welcome any other people's input too....