A motorway link from the M8 has been mooted many times
I can't see such a link being much help. If it were to be built, then during busy periods, the M8 in that area would end up being grid-locked even more often than at present. Outside peak periods, there's not much delay going via the A8 to the City Bypass / M8. IMHO there are many more deserving uses for scarce public finances than building a direct link between the M8 and Edinburgh airport.