According to the manual, the Leg mod is supposed to increase the movement speed of your agents, which is why I spent the money to research and purchase 4 of them. But after spending all that money it is clear that my agents are moving at exactly the same slow crawl speed as they were before. There is not even the slightest increase in movement speed, even if the agent is carrying a single handgun and nothing else.

So, is this a bug or what? The only explanation I can think of is that the game got confused because I installed the V2 Legs without first installing V1.

From memory, I don't recall the leg mod not having an effect on normal movement speed... but I could be wrong (I never really paid very close attention, and I have a horrible memory). So it could be a bug.

Have you tried making a group with one agent without the leg upgrade to see if s/he lags a little behind the other three in the group?

Perhaps the leg mod helps you move faster when carrying a heavy weapon (such as a chaingun)? EDIT: no wait, that's the arms mod isn't it.
Perhaps the leg mod speed difference is more noticeable only when you boost the movement adrenaline up?

Just throwing ideas out there :).

Try sending the two agents separately and see if one outpaces the other. I have a hunch the game moves grouped agents at the slowest agent's pace, so they more or less stay together. Don't know, though.

Even sent separately they move at the same speed unless you apply adrenaline. It's just a case where the person who wrote the manual was not well-informed enough about how the game worked.