There are a lot of people out there who will test something on a dyno and post a video or a screenshot of the readout showing HP and torque changes. Often, the tests are done to see how the car overall is doing after several modifications and tuning. But this thread is for collecting links to a smaller set of videos and posts that test individual mods. Some are good tests, and some are not good tests. But any video where they make a reasonable effort to control conditions as well as isolate the mod to reveal its effect on power output on the dyno is good enough for this thread. If this thread goes anywhere and a real list develops, I will come back to add an index to this post, with permalinks for results below.
First up, a pretty good test of a throttle body spacer by LHT Performance. LHT is a great veteran Honda modification outfit. I came across this test a few months ago. The heart of the testing begins at 11:40: