I wean between 5 and 6 months of age. I just play it by ear. If I see a couple slightly smaller animals who don't appear to be quite as growthy as they should be, then I hold the herd together for a couple more weeks. It may not do much, but it helps to not make me feel so cruel
As far as how much more quickly you'll get them fed out, its a tough call for me, as I don't know your area, forage, or supplement. My experience with my smaller framed Angus and roans (1200 lb cows) is that I will get 120 lbs more calf at weaning when creep feeding oats. Other guys with larger framed animals swear they're gettting 160-180lbs more calf. My average consumption is around 40 bushels of oats per calf for the summer. When I do wean, I don't see the growth hit as badly, since they're already eating good. That 120 lbs more calf carries through the winter, and with the reduced hit at weaning, I think I've got another 20 - 30 lbs more calf again. In -40, its easier to put weight on a bigger calf than a small one, so I'm getting some gain there too, but no idea how much. I'd be looking at my critters feeding out about 2 months earlier than if they didn't get creep fed.
These numbers are hardly scientific, as there may be a pile of variables in there that were unaccounted for in my records. I've always wanted to split the herd in half, creep feed one set of calves and not creep another set, just to see what I ended up with.
So given that you allowed them to wean themselves, you never got the growth hit that you'd normally see. So thats a wash.
Since your calves hit 1250 in 14 months, and I'll guess at 100 lb birth weights, you're getting 82 lbs/month growth. So if you get 120 lbs more calf at weaning, you should be shaving 1.5 months minimum off your total feeding time.
Rod