I started using new Starline brass instead of once fired Remington brass and got a noticeable increase in velocity. I don't have any experience with new and fired brass from the same maker so its not quite the information you were asking for.
300 grain Nosler
CCI 200 primer
Trimmed to 2.095
Once fired Remington with 55 grains H4198 yielded 2097fps average velocity.
New Starline brass with 54 grains H4198 yielded 2186 fps average velocity.
I believe that case volume increases in once fired brass, but I think there may also be a difference in neck tension.
On average, the Starline brass was 2 grains heavier than the Remington brass. The Starline brass also improved accuracy.
Starline will give you higher velocity. The tip-off is the increase in case weight. They make a stronger case, but that means less inside space. Less volume equals more pressure with the same load and thus more velocity.
The case question has too many variables. If your dies squeeze the case down to the minimum and the new case is larger, then the reload has the potential for less volume inside. Vice-versa would be true also. Then you get into things like overall case length, neck tension, etc. You would need pressure test equipment, chronograph, temperature gauge, barometer, etc. to make a conclusion that may change with the next day's test. I wouldn't worry about it in terms of hunting ammo and the benchrest gang will use just one case for all their shots. TALK ABOUT SINGLE SHOT RIFLES. :roll:
I neck size my cases. Actually, I found that the heavier a case of given brand, the larger it's volume. A 45/70 Rem fired case that weighed 195 grains held 273.2 grains of water, while a 192.2 grain case held 271.6 grains of water. Neck sizing the lighter case resulted in an increase of 1.3 grains, but the heavier one stayed the same.
Maybe it's spitting hairs. But I've seen a good load go bad with a slight increase or decrease in powder. So what would be the effect of a larger amount of volume? With my dies, F/L puts the volume back the same as new.