First, if you have a leather jacket, you are already faster (again I already said that), just dump some AP and use Sprint perk.......
Second, why trying to use logic NOW ? We are talking about people carrying different amount of stuff, with different stats.. Anyway the sniper can run with the sniper rifle on 1 hand only, and anyway it won't be more bothering to carry than let's say a super sledge ?
It looks like you're calling realism as your last argument, why would we set realism on this case only then ?