The unit speed is not directly moddable in a simple fashion. It is actually tied to the animation (although it does slow for fatigue.) There are some terrain modifiers that can be used to manipulate it. Kraxis has experimented with this some, as have others.

Scale issues are the real problem with respect to speed. Even if the unit speeds were 100% realistic, armies/units are of ~1/10th scale. Formation depths are usually no worse that 1/2 scale. This means that frontage is typically about 1/5th of what it should be. So lateral movements and such take far less time than they should (1/5th as much if the speed were accurate to begin with.) Honestly, lower than "realistic" movement speeds would be needed to simulate this properly. A true battle line could stretch well over a mile. You couldn't go running that length in 30 seconds. So to simulate the true "flanking potential" you need slower than actual movement speeds.