Fred,
I'm more interested in the currently available adaptors, tested against comparable factory heads/plugs.
The Norvel head is high compression, so I'd expect to see better performance against the Cox standard head, and you saw that in your test video. The Mecoa adaptors for short and long plugs are probably closer to the Cox #325 low compression heads in performance.
The Turbo plug heads don't interest me, because Turbo plugs are typically higher priced than standard or Nelson plugs, and I just don't see any advantage in buying yet another plug type.
If I'm not mistaken, Valentine heads are more expensive than either the Mecoa or Galbreath heads, and I don't know if they offer any advantage. Are they still in production?
Anyway, with Mecoa and now Merlin offering replacement plugs and heads, adaptor heads that do not give a significant performance advantage (like the Galbreath heads) may not be worth the expense.
Bill