Personally, I think it's more likely that it just enhances their ability with that particular metal, but I could easily see it demoting them as well.
Of course, the table says "In alloyed form, [Lerasium] produces various expanded physical and enhancement effects, (including the creation of mistings)".
I'm personally of the belief that there's a difference between a lerasium-dominant alloy (which probably makes misting) and a normal-metal-dominant one (which probably does the physical/enhancement stuff), but that's not really supported by much of anything.
Since atium+lerasium appears to be a nonstarter for becoming a feruchemist, my gut instinct is that alloying a feruchemically charged metal with lerasium could make you a ferring. No idea what would make you a full feruchemist, besides maybe a lerasiummind/lerasium alloy. I suppose technically Sanderson's never explicitly said you can only ever have one feruchemical ability, so it might just be a 'chuck every metalmind you can into it' deal..