You aren't taking any power away. The pump will still pump the same amount. If you closed the outflow all the way, it will just overflow the skimmer. For the noise, yes, just a simple cap with a hole in it works well.
How about the silencer? Any progress there? The reason I'm asking is once you do that, you will be adjusting again. When the pump pumps more air, it pumps less water. This can cause the level in the skimmer to change as well.
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