Just skip that idea man. I would estimate 50-90k easy plus cost of the car. You're talking about custom stand-alone engine management, custom fab, and somehow making it integrate into the OEM CANBUS.. aka nightmare. Would probably never be fully integrated.
It would be cheaper to rent a villa in Mexico and register a real RS6 there and drive it across the boarder.
A possibility would be to contact Heffner or someone who makes TT systems for Gallardos. packaging will be your enemy..