Don't give up. I have an identical setup that runs great so I am guessing you have some yet undiscovered problem. Mine had an APC 16x8 with OS F plug.
The Jett webpage claims 9600 rpm with the APC 16x8. I got slightly more than that. I don't remember what the rpm with the stock parts was but the Jett setup gave a healthy increase in power with very good behavior throughout the rpm range.

Have you tried a batch of fresh fuel?
Has the engine been run very lean? I saw one of these that had been run so lean the top of the cylinder liner had turned blue.
What tach are you using? The cheap hobby tachs we use can give unreliable readings if the lighting conditions are not optimal.