Calculated cooling system values.  They should not be treated as absolute; it is tough to match the perfection of theory in the real world.  However, they do provide guidance.

The columns are:

(outlet size in sq ft) (max hp) (mass flow in lbs) (velocity ratio exit/freestream) (drag in lbs)

The "eps" factor is a measure of heat transfer efficiency, ie how much you can heat the air in a pass through the fins.  Inlet sizes are also in sq feet.
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Stock RV-8 prediction, 0.3125 inlet, 0.25eps indicating average baffle quality:

1000ft - 100mph                                      8000ft - 200mph
.45   201.2  2.907  .686  4.2                     .45  348.9  4.685  .695  13.0

Vans styled the cowl for conventional inlet appearance and construction, then sized the outlet to cool 200 hp (the intended design maximum) at a typical climb speed.  Very practical and works fine, but the penalty is drag at high speed.  At 8000 feet and 200 mph this system will cool almost 350 hp, more than twice as much as it is possible to generate (manifold pressure loss, no turbo).  Outlet velocity is 139 mph (200 x 0.695).  The result is 13 lbs of cooling drag.

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The goal here is cool more than the designed hp value (210 rather than 200) with less drag.  To do so required work in three primary areas:

Increase eps to at least 0.30, perhaps higher.  
Seal cowl pressure leakage.
More efficient inlet.

6" round inlet (.3925), three different exit sizes calculated for two different eps values:

1000ft - 100mph                                       8000ft - 200mph

(stock exit)                                     		
.45  227.8  3.292  .776  3.4                      .45  394.5  5.297  .785  10.4  

(three smaller cowl exits)                          
(.25 eps) .35  196.8  2.843  .862  1.8        (.25 eps) .35  340.5  4.573  .872  5.3   
(.30 eps) .35  232.6  2.800  .872  1.6        (.30 eps) .35  402.0  4.498  .883  4.8
(.25 eps) .30  177.3  2.562  .906  1.1        (.25 eps) .30  306.7  4.118  .916  3.2
(.30 eps) .30  209.6  2.524  .917  1.0        (.30 eps) .30  362.1  4.052  .928  2.6
(.25 eps) .25  154.8  2.236  .949  0.5        (.25 eps) .25  267.6  3.594  .959  1.3
(.30 eps) .25  183.1  2.204  .961  0.4        (.30 eps) .25  316.1  3.537  .972  0.9 

Based on these predictions, I've built 0.327 and 0.250 sq foot fixed exits, swappable depending on mission.  0.327 sq feet should cool 210 at low speeds for typical fun flying.  0.250 sq feet should provide higher top speed and/or more efficient cruise, but will also require a higher climb speed to cool full power.  I may build additional, smaller exit panels depending on flight test results.

Even a 0.25 sq foot exit is excessive at 200 mph; in theory, given 0.30 eps, good cowl sealing, and a good inlet it can cool 316 hp.  Variable exit area may be a future project.