Yeah I have read of a guy shooting a 335 grain hardened lead slug from Cast Performance and getting good results at high speed with the Savage.
Another problem the guys have encountered with soft lead slugs and the Savage is failure to fire.
The guys will be at the range with, say, a 350 gr. soft lead slug, and 43 grains of N110, and all will be working fine.
At the end of the day, they leave a charge in the rifle.
Next day, they go deer hunting, and get a misfire!
The best guess is, that the soft lead accomadates to the rifling, and "relaxes" a little bit. This reduces the pressure. Smokeless needs high pressure in order to ignite.
If you want to mess with soft all lead slugs stick to a lower performance powder that has good ignition characteristics, such as 5744.