Has anyone found rapid fire for an SMG beneficial at all? Has it increased your KTD ratio at all or made any benefit to your game play?
Overall I don't think it is that useful due to the huge disadvantage of using up all your ammo way faster. However, this could be offset by using the Scavenger perk.
There are some benefits to it though, depending on the game mode and style of play. In Normal, you have the benefit of getting more hits on your opponents quickly, so a better chance of taking them out before they get a shot on you. In Hardcore, you get an advantage of sending more shots there way so a better chance of getting a quick spray kill. In either case it would be good to have Sleight of Hand (if you are an accurate shot w/quick reflexes) or Steady Aim (if you are a 'spray and pray' on the run player).
You're Kill To Death ratio largely depends on experience, skill, and smart play. It is hard to credit it towards any single Attachment, Weapon or Perk. But I have never heard anyone say they couldn't get by without having Rapid Fire on their SMG.
I would say yes and no.
For yes, you have quicker chance to down an enemy with rapid fire, depending on how quick you aimed at an enemy (best to have Sleight of Hand Pro Perk for this). You will notice when using rapid fire, you are likely to have recoil, some guns are awful with rapid fire unless you control the recoil by forcing to control the recoil with the controller.
For no, when using rapid fire, your ammo is more likely to be empty quicker, you would miss some bullets which empties your ammo faster, to overcome this use the Scavenger Perk.
The AK-47u with rapid fire certainly gives me recoil, even when trying to shoot an enemy from a distance that is harder to trying get most of the shot bullets into the enemy, I normally do short bursts when the enemy hasn't seen me. Even when using Lightweight, you may be running around faster, but when aiming and firing, the gun goes out of control (bad recoil).
I'm not mainly a SMG user, but I rather use the Famas gun with suppressor as when shooting, I find it quite steady when shooting, but who knows, everyone is different with their likes of using different guns.
I have had good luck using rapid fire and the extended mag if you don't mind using the warlord perk. The extended mag helps offset the speed that your gun will empty the magazine, and the rapid fire puts more bullets on a target faster. As far as benefits in general think of it as the more bullets you hit a target with the higher your dps is going to be, so the guns dps will actually be higher then a gun without rapid fire. You also have to take into account accuracy and recoil which will also be reduced with the usage of rapid fire. As a side note if you like to hip fire rapid fire can be quite devastating while hip firing. Since hip firing is generally random (you can only control the general area you shoot) sending more bullets down range at the opponent will naturally raise the odds hitting your opponent and also make those random hip fire head shots more common :).
The point of it is raw math: More bullets per second means more damage per second.. that is, if you can land all of the bullets. The purpose of this attachment is really just to try to get close quarters kills in a shorter amount of time. It's a tricky attachment to use because it reduces accuracy and eats ammo. Adding Steady Aim and an Extended Magazine or Scavenger usually mitigates this a little bit.