after the first few posts I was going to chime in and tell you that removing the pitot style scoop on the bottom of the cowl in favor of the snorkle in the the cooling inlet will actually cost you speed due to the loss of MAP. I just did this mod to my Rocket in the hopes that the loss of MAP would be offset by lower airframe drag. It was not to be. Looks like the pitot is going back on.
That said, your MAP is not dependant on a ram rise because of the turbo so the pitot scoop is not contibuting anything but drag (but it isnt much). I would not expect to see much, if any, gains in speed with the removal of the scoop. Airflow management and cooling exit size, sure.
Michael,
Evidence from Dan Chekaway (sp?) when he did a lot of testing on inlet snorkels was that if you do a good design of the snorkel intake that incorporates a filter on the left cooling ramp, you get the "almost" the same manifold pressure as a ram intake with a filter. The key is to have nice radiused lips around the perimeter of the air filter to improve the flow into the filter. You don't want the flow to have to turn around square corners.
Of course you lose a little bit of manifold pressure in the filter, and it is not easy to design a filter bypass for the snorkel intake like you can for a ram intake (Rod Bower, others).
My point is that the external diffusion in front of the cooling intake, plus a well-designed snorkel intake, can achieve "almost" the same pressure as a ram intake.
Sorry for the thread drift.
To address the OP's original question, the devil is in the details. We have seen as much as 7 kt of speed increase on normal Lycoming installations by putting a radius lip on the bottom corner of the cowl where the cooling flow exits, and reducing the exit size appropriately. This is achieved by a combination of moderate reduction of actual cooling mass flow, and a substantial increase in exit flow velocity. On the other hand, we have also seen a case where the Lycoming engine was baffled so closely, and the cooling was so efficient, that there was not much cooling flow to work with at the exit, and could not get much exit velocity. In that case, there was little or no speed increase, IIRC.
For your installation, the devil is very much in the details. If you can get good flow with little pressure loss through your intercooler and coolant radiators, and collect that heated flow into a well-designed exit, you will see very low cooling drag. If you have rather haphazard flow paths into and out of your radiators inside the cowl, and then expect to expel that flow with any significant velocity at the exit, you will probably be disappointed. Ross Farnham has been very successful with his Subaru installation using a fully external radiator system, because it is well thought out and executed. There is potential to do better if you can keep the radiators at least partially inside the normal aircraft shape, so that the cooling intake adds less frontal area. It would be challenging, but possible, to install radiators inside the norma cowl, and provide good internal ducting into and out of the radiators.