You did not mention that you were mulching, and that puts a totally different angle on things.
Mulching with a standard or universal deck ( does everything poorly ) is always a hit & miss affair.
The height of the grass after mowing and the actual length of the cut grass can make a drastic difference.
As will the temperature, he dampness of the grass and the type of grass you are cutting.
The airflow under a deck is a very complicated thing, which I for one have little understanding.
However it does require a lot more power to keep all the cuttings swirling around getting recut & recut.
So your original premice might be correct, a 10Hp engine will be working hard to do a good 28" mulch.
8/24's and 10/30's are an excellent size mower for blocks up to 1 acre if throwing or bagging but mulching might just be asking too much.
I run a lot of very old mowers 1960's & 1970's all around the 8/30 format .
I use these as loan out mowers at times like this when I have 9 mowers in the shop waiting for parts.
None of these will mulch. They all cut, throw and even bag quite well but none will mulch, the decks are just not deep enough.
About the only small RER that does a really good job of mulching I have come across was the 30" Toro Recycler and that has a 14Hp engine and a deck specifically designed for mulching.
Now my exposure to RER's is quite limited, they are not big sellers down here.
However those I own and service , that were not originally mulching, just do not mulch.
From what you have written it appears that you have done everything you can properly.
Dose it cut well without the mulch plug in place ?