Quoted:
negligible
the majority of blow back is through the barrel not the DI system.
A gas buster helps, a high volume can helps, but just a piston, not so much.
My 10" LWRC and DI guns are the same.
this has been my experience as well.
i find that the can makes a bigger difference than anything, really.
AAC M42k, gemtech HALO, YHM (whatever the .223 can is called) all have lots of blowback in my opinion, less with a gasbuster. i really couldn't tell a difference between piston or DI when shooting guns with these cans.
my surefire (and presumably other reflex style cans and other large volume designs) have less of a gas blowback issue, i don't really feel like i need a gasbuster for .223 with my surefire. i like it for 5.45 though. that shit's nasty. all of my uppers are DI.