I believe it would work better if the water entered at the bottom of the tank.
The original 1.6 gallon toilets in the 90's used a dam around the flush valve to prevent all the water from draining from the tank. Rather then use a quick closing flapper, like most do now, they installed a plastic dam that only allowed the upper water to flow over. Frankly those did very poorly.
But then there is marketing.
TOTO was the first to 3"
American Standard had their 3" with a wobbly flush tower.
Eljer then can out with their 3", and then Bain Capital closed those plants and merged Eljer into American Standard.
Kohler came out with 3.25" so their's could be the biggest.
American Standard put a funnel on their 3" and called it a 4"
Has anyone ever poured oil into their car engine using a funnel?
Did using the funnel make the oil pour quicker? Not for me. The funnel the gas station gives you is 4" at the top and 1" at the bottom. It pours out like a 1"
Does the water need to pour quicker?
Good question.
Kohler now has a flush valve less then 2" with a big canister on top which seems to work fine.
TOTO has the Aquia dual flush with a 2" and it works fine too.
There are so many composite features that go into a product that need to be tuned as a single component.
Changing the seal.