I'm a little confused. Hard to explain. If the engine was 300? with wind blowing on it, and then 300? with no wind blowing on it, how could you consider those two tests the same? I have to believe that the CHT would reach 300? much faster with no wind blowing on the engine. I'm not saying that the fan doesn't use up some horsepower, I'm only questioning how much.
I'm wondering about something else, if there's no air coming through the shroud, and the fan is spinning in a vacuum, is the rest of the shroud completely sealed off, you know, around the edges? Is there no way for air to get into the shroud? You know the atmospheric pressure is 15 psi and if there's any way for air to get in there, it will get in there!!
Now if you had had an EGT, and the EGT was reading the same on both tests, then the difference, if any, in horsepower, would be much more believable to me.
Another thing, during the length of the test, wouldn't one engine heat up much faster, and more, than the other? If not, I wonder what that tells you about the effectiveness of taping the shroud. (See previous comment)
I love this sort of thing, I wish I could see the data. I commend anybody who does this sort of thing because, the more data you have, the greater your knowledge and the greater your understanding.