The old Hemi was indeed a great engine. It was a great engine and it was a hemi.
As you say, the old hemi didn't start out as a monster. It was chosen by racers of the day because of its strength its size and perhaps because it was a hemi.
Much development went into that old engine over the years. It became one of the engines that was used by so many people that many manufacturers of speed equipment developed special parts for it and it became even more popular.
That sort of thing feeds on itself.
The Chevy small block is another example of an engine that followed the same path.
When I say that there is nothing special about the hemi, I mean that the hemi chamber will not get you much of anything just because it's a hemi.
There have been many very ordinary engines over the years that were hemi.
There is room for big valves in a hemi. Of course, you have to have big valves, not just have room for them.
If you design the head with great ports, you will take advantage of those big valves and make plenty of power.
NASCAR banned the hemi because it was an engine that did the things I mentioned above. It made more power than all the others. But not just because it was a hemi.
NASCAR is in the business of putting on a show. They will not let one brand run away and spoil the show.
American stock car racing is not a development class. If some brand has an advantage they will step in and make them even.
There isn't any reason that a hemi should have problems with emissions.
There isn't any reason that a hemi needs two plugs per cylinder.
So,
What I'm saying is that a hemi combustion chamber is good. But that in itself will not make an engine better than non hemi engines.