Just finished a two day IPSC match of 70 shooters, 9 shooting "Classic" with 1911's.
I was using 8 rd mags as the stages were designed for 10 rd mags (max allowed in Kanuckistan). With one rd chambered, that gave us 9 rds max, 8 rds on reloads.
Don't even go there re: revolvers as any pretext of "practicality" in IPSC has long since evaporated. At least IDPA has a "revolver friendly" aspect to their COF.
If you can count, don't miss and have practiced, flawless reloads, you can use 7 rd mags. However, once you cross the start line, you're already on plan B and any brain fart, malfunction or miscount is not going to add to your day.
Had I used my 7 rd mags with one up the spout, shooting to lock back before every mag change would not have been that much of a handicap. In fact, I planned my approach that way and was able to pull it off. Had I missed on a popper, it just meant another mag change.