Al and I agree regarding the shape of a Speedster in profile (that it is essentially an airfoil), and everybody knows that because of the shape of the thing, it's super-hard to get enough air to the (air-cooled) engine.
We disagree, however, regarding how to get more air into the compartment. This isn't facebook and we're actual adults, so we know how to disagree and still remain friends.
My way of looking at it is like this:
We go absolutely apoplectic when a guy leaves his tins off, because we all agree that there is a top and a bottom as it pertains to the cooling of the engines. Air comes into the fan on the top, blows through the cooler or over the heads and cylinders, and exits on the bottom. Between the two is a great wall of tin that keeps the hot side hot and the cold side cold.
I understand the arguments that air from the underside of the car up by the firewall is not the same thing as air that is discharged across the heads and cylinders, but no matter what you believe regarding airflow under the vehicle while in motion - it is the in the same cavity as the discharge of the stock doghouse air-cooler. At a minimum, pulling air from this cavity is pulling some of the reheated air from the oil cooler into the intake. Also, the argument presupposes that the vehicle has all of the sled tins (almost none of them do) and is always in motion and discharging air out the back of the car, which is not always the case. Several of the times I've come the closest to overheating have been in stop-and-go traffic.
I drank the Kool-Aid and put not one, but two giant holes in my firewall, and filled them with fans to force air into the engine compartment. The car actually runs hotter with them on than off, and hotter with the holes than before I had them.
Everybody talks about the infinite wisdom of the Sainted German Engineers (hats off, please), especially as it pertains to cooling - then advocates for the thing they most tried to avoid, which was pulling air from under the car into the engine. They pulled air from the top side. While the shape of the car is working against this effort, it's the only game in town. ANYTHING we can do to get more air into the engine compartment from the top is useful. The SGEs cut louvers in the Carrera decklid, and just added a second grill on later 356s.
Opening (or even removing) the rain tray is on my short list of useful projects. The IM/911 hole behind the plate is another method. I've seen one or two Speedster replicas with a second grill (like a 356C had). The Carrera louvers look cool, and can't hurt, but they provide very little by the way of open area.
Terry Nuckels reported that his car ran cooler with a luggage rack and a suitcase - which makes sense in that the airflow over the airfoil of the car is disrupted by the suitcase back there. Almost anybody who's driven where it's really hot will prop open the decklid by a couple of inches - I've had my oil temperatures drop 15* in the desert with this change.
It'd be nice to have some definitive, actual data, but the closest we've come it when Bruce did his manometer tests a few years ago, and they confirmed what I'm saying.
Regardless, guys (me included) are going to believe what they want to believe until there is conclusive data to prove otherwise. I'm going to continue to seal the top from the bottom.