lol, with 440hp or whatever and that small of a cross section, probably around 190 if the conditions are right and the road is straight/long enough (quick research reveals the C6 did 186-193 depending on which engine/transmission). My car with 430 will do around 175, but it has the aerodynamics of a brick. Thing is up at those speeds it starts taking an ungodly amount of HP for a relatively small increase in top speed, so it might take an extra 100hp to get 5 more mph, 200hp for another 5, and so on.