I took a closer look inside my estoril chamber while I have it cut up, there is no restrictor like this, but I think I found the reason why they seem to cut out past the peak of their power curve. They welded in a much larger degree cone inside the one you see on the outside of the chamber!
Taking this out should give me a more reasonably angled baffle cone and broader power curve, extending it on the top end, while lowering the peak just by extending the tuned length just a bit.