Will I have less POI shift with a screw on can adapter?
If I use a QD muzzle break will it be about the same as a screw on adapter?
I am using a Silencerco Omega, so I can go either route.

I do not think the particular attachment method would have any specific effect on POI shift. Only on way to find out, test with both. Due to the weight difference in the adaptors it is possible that would affect the POI. But I think the effective shift would be minimal as those devices don't weigh a whole lot.

I can see perfectly well how a can on the barrel "could" impact POI shift as the added weight will have an impact on the specific barrel harmonics as the gun is fired. The can does have significant weight to take into consideration. Again, only one way to find out. Test the rifle in the various configurations available to you and document/record the POI shift.

Lastly how does barrel harmonics affect accuracy. When a bullet travels down a barrel the barrel vibrates and the muzzle moves in some sort of pattern. Up, down, left to right, right to left, circles, figure 8 etc. This can be seen using high speed cameras. When you put some sort of influence on the barrel ie making it longer, shorter, thinner, stiffer, resting point on a sandbag, you will influence the precise location of the muzzle in its movement at the point in time when the bullet exits the bore. This is similar in concept to how a guitar string changes it's tone when fretted in different places on the instrument.

According to the accuracy gurus one of the things you want to minimize is all of the variables in the entire shooting process INCLUDING the departure point of the bullet from the muzzle. Therefore the presence of a can on the barrel has the "potential" to affect the POI. The degree as to how much of a shift [to my knowledge] in not predictable. It can be tested and determined on the range, but not quantifiably "PREDICTABLE". I am sure someone with far more ballistics experience will come along and either confirm or correct my position on my poor attempt at explaining this. (Not being snarky, just recognizing the limits of my ballistics knowledge)