The other issue with steel in vintage guns is pressure. If your vintage gun isn't rated for modern SAAMI-level pressure, then you have another issue to deal with in addition to the hardness of the shot and the choke. There are CIP-approved steel loads for their "standard proof" guns, which have a somewhat lower service pressure standard than under our SAAMI standard. They claim that those loads are OK for old guns (assuming good condition, still in proof, etc). However, I'm not sure I'd want to risk it. And as far as I know, those CIP standard proof steel loads aren't available on this side of the pond anyhow.
