Did I read correctly and someone was using two different barrels to measure accuracy differences in threaded and non-threaded?
All that does is tell you the difference between the barrels...
As far as threading a barrel, the amount of material removed may cause a slight shift in POI, but probably not enough to notice. If you add a thread protector to cover the threads, than that may cause a little more POI shift.
Most likely nothing will happen to group sizes.
A suppressor will most likely cause a significant impact shift. If the suppressor indexes to the same spot when removed and reinstalled, the POI shift should be consistent in its change, and that can be corrected for. If not, you would have to leave the suppressor on all the time or re-zero every time you installed it.
As far as group size... A well designed suppressor should not affect group size adversely, or very much if it does. Poor designs can creat turbulence that affects the bullet in different ways every shot, which opens up groups. There are also several variables involved, so even a good suppressor can cause issues with some rifles, so trying a different one may be needed in that case.
A 30 caliber suppressor will work with a 5.56, but it will not be as quiet as a dedicated 5.56 can.
Also, the thread sizes will most likely be different... As the 30 cal can uses larger threads, meaning an adapter would be needed for the 5.56 rifle, unless you had the barrel threaded the same... But seeing as this is most likely for your AR, that isn't possible, as it is already threaded.
I would say the best bet is to get a dedicated 5.56 can for your AR, as you can get versions that use the existing A2 flash hider for quick connect/disconnect.