Well, I'd be careful to never say never. Rogers bullets are BHN 21, just one point south of linotype. That's the same hardness as Beartooth bullets, which are driven to 1800 fps in .45-70's routinely (though Beartooth gets to their hardness by heat treating rather than by alloying). By comparison, Elmer Keith developed the .44 Mag with 16:1 lead:tin (BHN 11; he actually started with 20:1, which is BHN 10). If the lube is good and the gun is smooth inside, it might surprise you how fast those can go.
At the same time, you bring up a good warning to watch for lead building up as the load is developed. Like pressure, it would be another warning sign that you need to back down. Lead can accumulate until pressures are unacceptable if you run a load that leads heavily. Not to mention losing accuracy as the lead layer grows.