It is significant, it is measurable, it can and does mess things up. Inconsistent bullet grip, runout issues, et cetera.
Big Time. And worse.
Here's an example and my first experience with the donut.
I formed 7 Mashburn Super brass from .300 Winchester Magnum brass. I had previously neck turned the new .300 Winchester brass before forming the 7 Mashburn. After running it through the die, it was obvious that the shoulder was pushed back and the neck of the Mashburn was longer, now comprised of part of the shoulder of the original .300 WM brass. I had turned the .300 WM brass down and into the shoulder a bit as I always do to actually avoid forming a donut.
The donut was obvious, but I thought it wouldn't matter since it was going to be on the OD anyway after sizing with the expander ball or expander mandrel before doing any bullet seating. I proceeded with load development and really had a difficult time; I couldn't find an obvious node in the ladder. I tried a couple of loads in the ladder and shot them for long range groups as well as 100 yard groups.
Long range groups were horrible with tons of vertical. All over the place. The 5 shot 100 yard group was pretty good.
Thinking it may be a seating depth issue, I went out and shot a test. It didn't improve anything. 550 yards with well over an MOA of vertical stringing...
Then I got to thinking about the obvious donut at the base of the neck on the brass. Threw some calipers on it and measured it. Checked the reamer diagram for neck dimensions. Chamber neck dimension was .317" and the donut at the base measured .317" to .321". Compound that by seating a bullet in the sized case and pushing it out another .002" and you have a big problem chambering a round. The bullet was being squeezed big time when the case was chambered in that little donut area.
The real telltail was taking a fired case and slipping a bullet into the neck. It should slip in freely all the way. When this was tried, the bullet would stop in the neck when the bearing surface in front of the boat tail came in contact with the donut that was now in the ID after being fired.
I ran the existing sized brass through the expander mandrel, neck turned off the donut all the way into the shoulder a tad, loaded 10 and went to the range with the load I had.
Vertical was cut in half and accuracy was much better.
I'll have to reshoot load development because I'm sure the results were inconclusive due to the ff'd up brass.