The rifle as-is is a very powerful weapon, and there is no recoil, literally.
Making the rifle any more powerful would be heavily unbalancing, as much as I would love it.
Humans really don't move that fast, considering the movement needs that humans have.
If humans were slowed to match the speeds seen in the ever-so-common "realistic" shooters, it would absolutely cripple their abilities to fight.
Tremulous is more based on movement than aim. If humans were slowed and the rifle were powered up, it would encourage camping at the ends of hallways and really just create a pretty boring game.
As for flashlights and shadows, I believe that is a technical limitation rather than a design decision.
It's been discussed before, try using the search button.