I would agree, that if you have the cash, get the surefire forearm but make sure it is the model that includes the swith to allow you to disable the pressure switch. It is possible to accidentaly bump the pressure switch and give your position away before you would want to. I'm talking about situations where you are investigating those bumps in the night.
I do own an insight streamlight M3 and have found that because it mounts close to the end of the forend, the barrel protrudes past the light thus casting a partial, but light, shadow in the beam. Contrary to what Quattara asserts, the beam itself is not flawed and is very bright. I pointed the light into a mirror just to see what the light looked like pointed at me from a distance and succeeded in blinding myself for a split second. In addition, because of where the light mounts on the 870, the remote switch is not necessary, the light can be operated easily with the forend in the forward, chamber closed position.
All I can say about the surefire is that because the light protrudes forward from the forend, the barrel won't cast as much of a shadow in the beam if at all and all of the switches are right on the forend itself making it even easier to work the light.
I think I would have gotten the surefire if I had the cash myself, but I am very happy with my M3