Accuracy improvement over what? A2, A2 FS?
If that's it, yes. A Vortex (or for that matter, a KAC M4QD or the Surefire FA556A FS) will tighten up groups.
On my 16" SS MRP, you could see group sizes go from 1.25" at 100M with A2 to Sub 1" with the Vortex or M4QD. IIRC, the Vortex groups were a tad smaller. Same day, same range conditions. On the concept gun, the SF FA556A and the M249 Vortex had the same effect in comparison with an A1 FS. At 225M, I think the group sizes went from 2.5" to 1.75".
All groups are five shot, and with an accomplished (sure as hell not me) shooter behind the trigger.
I think it has more to do with how the FS is installed and the consistency of the machinework than the actual FS itself.
With that being said, when I shoot I can't tell a difference, but I suck.
-Cap'n