Badger - have you tried one of the moly preps? MsMoly and Hoppe's moly bullet sprays might aleviate the leading.
Apparently, the Gualandi slugs are not only very soft, but gas blowby will cause soughing off of lead ahead of the slug's travel onto the bore and the slug and wads then iron it onto the bore when they pass over the 'particals' of lead.

Spraying the slugs with one of those moly's might work - or they could be tumbled with moly powder as sold for that purpose.

Of course, personally, I prefer RB's - but - I do have some Lyman sabot slugs at 506gr. (WW metal) that fit normal red WWAA field 1 1/4oz. shotcups for testing & I do have some IMR4227 for loading high vel loads, along with Herco, 4756 and 7625 - + some others for standard 1,500fps loads.
Testing as noted on Ed Huble's thread on 12gauge FH showed the lyman's doing very well accuracy wise from 1,500fps to very much higher speeds.