Keep trying until it becomes worse again! That could even be f/32 or beyond.
You're right to be suspicious about the ultimate quality of the Raynox.
Here is a test Rik ran which compared a Macro lens + lower power Raynox against a better optic, and the difference is clear. You are pushing things further with your stronger Raynox.
I see they cost about $135. For less than that you could buy a good 10x microscope objective, which would outperform it by a good amount.
Eg in the FAQ Here
Your Raynox would be good though for videos of small things, or with a small-sensor camera. With those you often need to be hopelessly close to the subject to get "macro", at the wide angle setting. With this Raynox you could use a longer zoom setting on the camera. The perspective might reverse, which you may not notice!
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