Gun suppressors are a cool thing to have in games because they keep you quiet and stealthy and are lots of fun to sneak around with and take your enemy by surprise. Most games introduce a damage or range penalty to suppressor equipped guns to compensate for that extra sneaky lethality.
From the research I've been doing and the help of some friends who've contributed their experience(thanks +Brandon Fero for your insight on this), suppressors don't really affect the bullet speed, range, performance, whatever of a gun. What does happen though is that bullets usually get changed from supersonic to subsonic. What does this mean? Well the bullet speed as the projectile leaves the muzzle is controlled to be below the speed of sound. This reduces the sonic crack that's distinctive with supersonic ammo. Getting confused? This video will clarify it with four rounds being discharged from a Desert Eagle, two supersonic and two subsonic.
Notice how the supersonic bullets still produce more noise? That's because the bullet is creating a sonic boom just like a fighter jet. Firing subsonic removes this noise and makes the overall performance a lot quieter. The issue is that supersonic bullets behave differently from subsonic, thus the weapon's performance difference when suppressed. That is, suppressed and using the slower subsonic bullets. After all what's the point of the suppressor if the sonic boom gives you away?
Now slower bullets drop faster thus altering range. They have less momentum (p=mv) and thus theoretically less stopping power. Some articles mention that subsonic expansive bullets will penetrate more since they expand slower than supersonic ones. This is a good thing as they go deeper and may cause more harm, even when they have less momentum.
Overall the point is that suppressors don't affect the performance of the weapon. If your weapon of choice in the game already has subsonic ammo the addition of the suppressor should be transparent.
How does this come into play in the game. Well weapons already have a modifier for range and ammo is getting a penetration and damage modifier too. So those will be put into play when using different ammo as you switch to using a suppressor. Performance will depend mostly on the ammo being used in the weapon. There are already a lot of variants with ammo being taken into consideration that adding this should be a non issue.
One thing I have to take note is the supersonic to subsonic transition and its effect on bullet trajectory. Specially for sniper rifles at long range. Using subsonic bullets prevents this, but I guess it will reduce range considerably. Last time I read something about this I got the impression that the transition point pretty much marks the effective range of the weapon as the trajectory becomes too unpredictable to make the weapon effective beyond it. I'll have to dig into that again.