The max damage might be comparable, but the splash is much smaller than a thrown frag. If you stood three soldiers shoulder to shoulder and shot center of mass at the guy on the end, you'd probably kill two with little or no damage to the third. I've definitely killed multiple targets before (especially around crowded sundies), but generally it's only reliable against one. Note: I've never tested it as I described above, but it seems easy enough to check. I'll do it next time I have a few friends online, but I encourage other people to conduct some experimental teamkilling as well.
No, the radius of the damage from the grenade launcher is very small but lethal. With practice you can gib 2 enemies if they bunch up. I use the flash grenade instead of frag. Its easier to lob the nade into the room, run in while they are blind, drop a c4, run out, "Click".
