Does this mean that loading the case (more) full would lead to (less) unburned powder?First, many of the powders used in pistol loads are older technology (Bullseye, for example, was first sold around 1895 give or take) and tend to be 'dirtier' than more modern powders. Most such powders tend to burn cleaner at the high end of the pressure range, but still have significant residue. Second, two major factors affect the burn rate in any pistol load: bullet pull and bullet weight. The more bullet pull (neck tension) you have the better off you are in terms of burn rate. And, assuming good neck tension, a heavier bullet will always raise pressure which also promotes better burning. Third, bullet crimp is vital. To the extent possible with autoloaders, the more crimp you have the better off you are. Any additional delay in the bullet starting to move, increases pressure and improves burn rate. Finally, fourth, is primer choice. If you've done everything possible to improve ignition and burning, and still have problems, use a magnum rated primer to increase initial internal case pressure at the start of the powder burn. One of the problems with small charges of fast powder is simply the amount of air space in the case versus that occupied by the powder. The bigger the case, the more likely you are to have ignition trouble with such powders. The .45 Colt is infamous in that regard. So, as a final bit of advice, change to a different powder that fills the case at least 80%, 100% is even better. That is the best solution for the .45 Colt for instance, regardless of the speed level you are looking for. A full case (powder space under the bullet) means there is immediate ignition of the charge since there is no air space to lower the brisance level when the primer fires. That promotes consistent pressure from shot to shot, which in turn means higher overall pressure to start with for a better burn, and much tighter ES/ED numbers.
I would have thought exactly the opposite. Luckily I'm loading very moderately, so there's plenty of room to experiment. The bummer part is I already loaded a thousand rounds like that. Ain't no way I'm pulling a thousand bullets.