Atleast in germany you may only overtake a driver if you can do so without going over the speed limit. I would assume that this is that case for most western countries too, driving over the speed limit is illegal, if you do it while overtaking I see on reason why you should get an exception.
And after all, it's a speed limit, not a speed average or speed minimum.
[NB Reformed speeder here - I used to speed a lot, but then got a ticket and decided to stick carefully to the limit at all times, which actually means less stress, less fuel used and I haven't noticed arriving places really any later. Have almost got in fights a couple of times though!]
Even if you go into the lane of the oncoming traffic, you are still not allowed to break the speed limit.
If you're not part of the police, firefighters or ambulance drivers, I do not believe there is a single valid reason to break the speed limit.