Most airliners already cruise at a speed that is close to the maximum cruising speed, and although there is still an element of margin available, it is probably much less than you realise.
I always understood this to be the case, as it has to do with efficiency of the engines, hence the familiar announcments (especially on transatlantic flights) equating "tailwind = early arrival" and "headwind = late arrival".
However it seems that it doesn't apply on short-haul. On a few recent flights I was pleasantly surprised to hear announcements such as "80 knot head wind" coupled with "twenty minutes early arrival". OK we were 5 mins early leaving, but how did the pilot gain the extra 15 mins (on a flight less than an hour)? The answer seems to be that the flights were scheduled at well below the aircraft maximum speed - the figure I remember hearing on the occasion mentioned above was 325mph, which is well short of the maximum speed for the 738.
Not surprising that the airline concerned can boast a magnificent punctuality record with that much speed in hand!
Cheers
UFO