I say it would be a bit like a bell in that it has extra volume, being of larger diameter and the exhaust would still be moving slowly compared to when it narrows at the vertical chimney. I bought the tubes today so I hope I'm right...
I did read the post to the end as recommended and was thoroughly entertained, I like the science fair interlude.
Is that ytong block wrapping the firebox? I was considering using it until I dug deep and spend a staggering quantity on ceramic fibreboard.
Here's a question.... To make a slab for the top instead of steel, can I cast one like someone said here with clay and sand and arlite? I can't find a refractory slab around here.... And, why arlite? Isn't it insulating, rather than accumulating,,? Also, I'd hate to see it collapse and crush my fibreboard riser :-p