I truly doubt that anyone forges small internal or external parts. If they did and sold them for a resonable price, they'd probably be supplier for the whole industry.
More likely parts like slide stop, thumb safety, spur hammers, etc, are cast and then machined.
Some parts that you can stack like ring hammers, sears, etc are wire EDM cut, and then machined if necessary.
What matters also is the heat treatment, and quality of steel used.
MIM parts are popular because they require very little final machining. IMO the jury is still out on MIM parts. No one ever did (publicly) any impact, tensile, hardness and wear tests on MIM vs. other parts. They are inferior, but how much, and is it acceptable? Yes some people that offer "Kimber upgrade packages" etc, have bitched about MIM parts, but that's part of their marketing. I guess street testing (shooting the guns) will show when there are 20,000 Kimbers out there, with 30,000 rounds shot through each.
If you want to play it super safe, get tool steel parts with best available heat treating. Then have them frozen, which changes the grain structure helping to reduce wear.