With the short running that you are doing and the high speeds your are seeing i would go twards the streamlining "as areo at high speeds are detrimental as you know" and keep the engine tucked. With your small duct you should be able to get enough air in there to properly cool the engine enough for one high speed pass. Now if you were heat racing it would be a whole other story.... I would say expose that engine so it can dissipate that heat!

Look at the guys that are drag racing for example... There doing something similar to the speed runs. "1 pass top it out and shut it off" Your just going a little further There making 1 high speed high power pass and for weight reduction among other reasons will chop the cooling head all the way down. There not running for an extended period of time so they don't need the extra cooling or the extra weight. They want the heat to get up as quickly as possible for the engine to make its power and make that fast pass and shut it off. But since you are going a little further and seeing more load I would keep your cooling head on of course, just keep it tucked.

As far as temps every engine is a little different on what they want but i wouldn't let it get over 270s on the temp to make sure to not over heat the engine and cause damage.