These were only fireformed cases and pre turned with a bite into shoulder. There were freshly annealed and turned to .0135 wall for a 338 Neck. They did not have a donut and we only size 3/4 of the neck with a bushing die so that part of the case would not have touched the bullet. Now this is a 17 lb. light gun for 1000 yard BR and we did the same with 2 others. WE throated them from a .210 freebore to about .290. These are all tight necked guns with brass turned to one ten thousandth of an inch so it is not a thick thin brass thing.We also did the same thing with a dasher and it left it left us get to 3025 With 105 VLD's. The pressure went away. The dasher was a 6BR case turned to .010 with a .266 neck. The neck was turned to about 1/16 inch from the case neck junction. Now when fireformed the .010 neck goes back to the middle of the shoulder. No donut. The loads went from 32.5 with pressure to 34 grains and no pressure. A 210 Berger in a WSM with H4350 which is the powder of choice for 1000 yard benchrest usually gives about 2850 tops with a fast barrel. The case is touchier then you think. It goes from good to bolt click in .2 grains of powder. I have shot these two cases for over 6 years now and they are the 2 to shoot for accuracy. They hold about every record in 1000 yard Benchrest. They also win about every aggregate at every 1000 yard range in the country. In Heavy gun I shot 10 shots in 2.815 inches at 1000 and it is the current world record in heavy gun. MattI would say you got the increased velocity simply because you removed the pinch point between the neck of the case and the bullet and had very little to do with the longer OAL and the additional two grains of powder used.
In fact, I would bet that there is a minute donut at the base of the inside of your case necks that was causing more of a problem then the pressure ring on the berger bullets which is very minimal at best.
This is one reason I do not design my reamers to have extremely tight necks. Can cause these little issues that you saw and the more extreme you get in performance the more you will see the effects of these things in velocity and pressure. In the 300 WSM which is about as mild and user friendly as it gets, you do not see much of a problem. In something like a 7mm Allen Magnum or 338 Allen Magnum, it can be dramatic and cause severe pressure swings.
Again, I would bet that if you inside reamed your case necks or had a neck that was 1/2 to 3/4 thou larger in diameter, you would not see this problem with pressuring up. I believe you simply removed the pinch point and that resulted in freeing up some extra performance, very little to do with the actual longer OAL and two additional grains of powder. In my opinion anyway.