Open coupe, 289, about 145mph, .41+ cd.
Lemans coupe, 289, with roof, about 155mph, the roof helped a bit, without adding frontal area, and reduced drag, maybe .38.
Daytona Coupe, 289, tagged on Mulsanne at over 190mph, about .24.
Daytona Coupe, F5 (closer to the Italian design,) 650hp plus, 216mph.
The difficulty is that to go ten mph faster, you need double the hp, or reduce the cd by a lot more.
Tilt the windscreen, some help. Track roadsters were commonly slanted back as much as they could get away with. Remove the windscreen and add a small bubble, significantly more. Lower the car, go faster, add a dam, splitter, and tray under the nose, even more. Recontour the rear trunk and control the adhesion of air to keep it on the deck, faster yet.
A good example is the Audi TT, which at speed had some serious instability issues. They were recalled and a wing/spoiler added by the factory. Cute design, horrible aerodynamics. The problem was the center of air pressure on the car being ahead of the center of gravity. It pushes the nose around and it resolves by swapping ends.
Look at the LSR cars at Bonneville, the 249mph Studebaker coupe, for example. Very similar frontal area and cd compared to the roadster. They added a wing with side boards to increase the swept air surface on the side, and control the air coming off the roof and tail. It keeps the center of air pressure behind the center of gravity and makes it more stabile.
The Coupe with more aero work underneath and with a different rear air device should go a lot faster with 700 hp. The traditional spoiler was an expedient piece of aluminum tacked on at the last minute at a race to generate some vortex and reattach flow along that sloping roofline, plus enhance the affects of the Kamm tail. As designed, it needs more work to reduce lift overall. The side profile is still a wing - air has to go over it faster than what gets under it, and it creates lift. The original wing would have likely worked better if they had stuck with it and worked it out, even if it was tilted into a stall.
Here's some web sites to peruse:
Bonneville and Land Speed Racing Links
Just one article of dozens here:
http://www.autospeed.com/cms/A_2159/article.html
The entire history of the Daytona Coupe is basically the understanding by Shelby American in the day that they simply couldn't keep up on long Euro courses, even with 2 more liters of engine displacement. The result was the Coupe, which ran the same motor as the roadster, but ran 40mph faster on the top end.
Essentially, free hp because they didn't have to push a wall of air in front of it. The concept is reproducible and proven, lower the cd and you go much faster with the available horsepower. It's where that speed is needed - above 100mph - where the most engineering has taken place. Which leaves out the bulk of us. We simply don't go that fast due to a number of reasons, so we have little to no familiarity with the concepts involved.
And since the factories can't sell gas mileage to us yet, we also don't care. But it's starting to change. Wait for 2016 and the mandatory 33.5 mpg the CAFE jumps to, aero will get popular again because we will be buying it like it or not. Expect serious reductions in frontal area and a lot more attention to airflow.
So, is the roadster a barn door, yes, emphatically. Shelby knew it, and only saw success with it on short tracks that emphasized acceleration, not aerodynamics. When it came to top end, the roadster was a brick, and there was no doubt about it, it had to go. The Coupe won the GT Championship, not the roadster, something the 427 S/C fans don't like being reminded. Hence, the fascination with one lone 427 making it over 190. It takes another 2 liters of maximum power to push it that fast. The Coupe did it with a 289.