Well, that is not a "can", that is the Noveske KX3 "Flaming Pig", of which i am not a big fan. It is a "blast Re-director" that is VERY loud and adds quite a bit of weight on the very end of the muzzle . However, the fireball out the end is pretty cool at night. Does not do much for Night Vision (read obliterates it), but if you like it, by all means put it on.
As for the weight difference between the 9" with FSB and 12" with Low-Pro, it is a pure wash. The 12" just makes the barrel "look" even shorter. the 10" will JUST cover a small Low-Pro GB (Pri, Vltor, YHM, Etc) but not a Shaved FSB (on a middy), you would need the 12" to cover a shaved FSB.