cw, just a couple or three comments. I also think headspace is something one should know. As I mentioned,I measured it on a bunch of guns. Unless you have a grossly under length case, and/or a chamber grossly over length, the case will not headspace on the extractor. This stuff about the extractor holding the case has been repeated so often, for so long, many shooters are sure it must be true. I have found that the extractor will on average hold a .45 ACP within ~.035" of the breech face. So if someone has a pistol that is actually headspacing off the extractor, something is very wrong.
As to neck sizing only, I wouldn't recommend it with a straight sided semi-auto pistol case. Been there, done that. The case will expand on firing near the base, especially in the unsupported area of the barrel throat/feed ramp. Unless you're are sure you only shoot the reloaded cases in the same pistol and make sure the most expanded part of the case is lined up with the barrel throat/feed ramp, 100% reliability will be less likely.
I have experienced the case shortening on firing. As with measuring headspace, I've measured a bunch of cases, before and after firing. Measuring new cases, I've found they may shorten several thousandths upon firing. Then after resizing, the cases will lengthen again. I've seen changes up to .007". As there is the same amount of metal in the case before and after firing, I suspect that as the cases expand radially, the length is decreased. When I resize the cases, the case is squeezed radially and some, but not all, of the original length is restored. Not back to the same length because the base of the case is not sized down back to factory diameter in standard sizing die. I've found taper crimping to be very beneficial too in achieving that 100% reliability...