Very interesting. I've just finished reading Sigrid Rieuwerts' 1991 paper 'Historical Moorings of 'The Gypsy Laddie'. Of course she makes connection between various Cassilis earls and the Faas, but doesn't go back as early as this. Good luck in your researches and please post your findings here.
Child cites Pitcairn, Ancient Criminal Trials in Scotland (3 vols edinb. 1833) which might be worth a look around this 1510 date.

Like Matt I would also be interested to know where you got your primary info.