Re: AR barrel length question
As Glock24 posted, the weight reduction will not be significant, and would not be sufficient reason by itself to do this IMO. I have both a 16" Colt and a 14.5" Noveske (with permanent Vortex) and prefer the 14.5" upper for its shorter OAL and maneuverability inside structures.
If your 16" upper has a carbine-length gas system, reliability should not be adversely affected. If it has a mid-length gas tube, however, the gas port may or may not be optimal for the shorter barrel.
There are some maintainability issues for permanently-attached flash hiders that should be considered (i.e., inability to remove the FSB / gas block without drilling out the pin/weld). It's best to send your upper to someone who routinely does this sort of work.