On nearly all guns, it only increases recenter speed by 2%, which is extremely small, and it doesnt reduce recoil (scar-h is the exception, grip works on it). Drift0r has put out a couple helpful videos about what exactly it does, id recommend watching them.
Long barrel is likely more useful on SMGs but grip does come in handy for those really long range gunfights. You'll notice it more on AR's.
Grip makes a huge difference. I'm a rusher who uses SMG's all the time. When I rush and I spot a target on hardcore without a green/blue name, I auto-aim and turn and shoot. Grip helps me land my target. Without it sometimes will cause me to miss because it doesn't aim and connect in time. I usually run with a reflex sight, quick handle, and fast mag with my guns though.
no it doesnt and personally i wouldnt waste a attachment slot on it and that goes for every other gun that has a grip
Are you sure? I'm fairly sure it helps a lot on the Skorpion as well. As you said earlier it doesn't help recoil, but I do feel like it makes it easier to control.
Yes it helps.
When ranking up a gun, I use it when there is nothing else but quickly replace it with Stock, Quickdraw, Sights or just about anything else. I don't think I have ever tried it on a SMG.