You touched one the points, its drag. Aston Martins have a beautiful shape but they have an incredibly high grag coefficient. The DB9 for example has a 0.35 , the Vanquish has a 0.32 and since the AMV8 is the same shape it should be in that ballpark, though if it has a slightly smaller frontal surface area it will help the overall drag.

Weight is not a problem since its not a big factor in top speed, which is mostly about horsepower and drag. Given that the 996TT has a smaller drag coef & smaller frontal surface area (overall drag is = veolocity x frontal surface area squared x drag coefficient) and it has more HP than the AMV8, then the AMV8 should have a significantly lower top speed. Since the 996TT's top speed is 189mph then in my opinion the AMV8 should have a top speed right on the 997S's range, 182mph or less, since the drag is more important factor than top speed (a small change in drag such as making the side mirrors smaller has a greater effect than a small change in HP) and the 997S benefits from a much smaller drag vs the 25 HP extra of the AMV8. IMO they will probably be within one or two mph of each other