The exact reason why Sirens can be so deadly, particularly since they tend to hide behind other Zeds. If the advantage is really that ridiculous, Sirens would be gone by now.
A ranged attack Specimen would be really balanced in fact.
The Siren's attack is so lethal simply because it's an AOE effect that GOES RIGHT THROUGH YOU, YOUR BUDDIES AND THE ZOMBIES.
A specimen who's got a gun strapped to him could easily be balanced by the fact that his allies would block his shots (IE, he has to be exposed to be a threat), he can only shoot one player at any given time, and he can't damage you if you're not in front of him because guns, unlike the Siren's scream, are not 360 degree attacks.