Just wonderiing.  My guess is that it does.  Is it tolerable without a gas buster in this case?

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.

I have a 5.56 YHM and the blowback lifts my eye pro up. I almost dont like shooting with it.

Gas buster helps a little but still have the same problem.