Maybe I got lucky, but the trigger pull on my new M&P15OR is awesome. It's probably crisper and smoother than some trigger work I had done on some of my other rifles. I haven't scaled the trigger pull yet to see what it's at, but I really like it. It's not a 3 lb pull (Which is too light for me anyway); but it's also not a traditional 7-8 lb pull like one might expect. I'd say it's probably around 5 lb. A trigger job is usually one of the first things I look at doing with a new rifle. Either myself (DIY) or paying. When I was buying my M&P15, I was already thinking and planning for that. After dry firing, I was impressed; but realized that isn't real life. Well; I've only had the gun for about 2 months (Bought it at christmas); but I have about 800 rounds through it. I have decided that I have absolutely no desire to even touch the trigger on this thing. It's simply too nice.
As for a drop in trigger; the question really is: Have you ever had trigger work done before. You're the one who knows how smooth and light a trigger needs to be so that you can hit your target without too much muscle distraction. I know that when I have a trigger that is too light, I get self conscious about firing unintended. My mind spends more time on the trigger than the target. I personally like having my finger resting on the trigger while I'm aiming. So, the question really is: You say you want to tighten up your groups. How are you shooting now? Triggers themselves don't improve shooting. They simply make you the shooter more relaxed in your handling of the weapon. But unless the trigger is loose, sloppy, heavy, or some other noticeable distraction, I doubt that a new trigger is going to help.
