The bleed off of gas from the bushing is very small, besides the bullet is out the barrel before the powder gas diverted from through the bushing blows back the op-rod.
So while I haven't chrono'd different bushings, I would bet money you'd never notice a difference.
You won't notice a change in accuracy between bushings, but the process of taking your gas block apart, changing the bushing, and putting your gas block back together, will show a different point of impact, unless you are very lucky and get the exact same torque on the gas block screws and exact same gap between the halves of the gas block. I usually need to re zero after changing bushings.
Last edited by sandog; January 7th, 2017 at 05:38 PM.