VL
You are right in saying that "best rate" and "best angle" are two different things. On a 757 best rate is given as Vref+140kts and best angle is taken from the FMC (about Vref+80kts).
In reality, when told to expedite I don't usually increase the thrust, I just dial the speed back towards the best ANGLE speed. The reason for this is as eyeinthesky states. Most radar controllers would like us to achieve the required separation over the shortest geographical distance rather than the shortest time.
In the case of descent, however, we just dial the speed up as high as we can. We may not even deploy the speedbrakes. The increase in ROD at 330kts is quite maked over that at 270kts!
G W-H