The "mid range" on the engine in the video sounds fine. During most of the video the engine is at what I assume is full throttle, with a little idling and a bit of mid range stuff (when your tach shows about 7K) which sounded fine.

It actually sounds like the engine is overheating to me - it's sagging back from a peak of somewhere in the mid elevens back into the tens once it gets hot. Which prop were you running in the vid?

I would lose the ally tape and try the run with all else equal again.