Hit/Dam - YES
/Saves - zero returns
/Ac - YES & NO
Hitroll and damroll always had diminishing return for a long time. For example having 80 hitroll could mean you actually only have 60 (not actual value) for parry breaking computation.
Saves always used the D&D saving mechanic principle.You stack svs vs all + svs the spell school and roll against it. 1 you fail, 20 you autopass. So having a huge modifier gives zero advantage as a lower one after you pass the break point, unless you get debufed by saves erroding spells.
But Morlarch changed the saves system, I think it's still the same principle, but only he knows for sure. Still the recomended saves is 30 across the board. 30 vs spell + 30 specific.
AC - AC didn't use to have diminishing returns. It had enhancing returns. Each point of AC always provides more returns than the last. But it was pretty hard to stack it very high.
But Morlarch changed it. I can't find his post, but he said he changed so that after an value we now get diminishing returns. Like 800 ac is old 600 ac. Or something like that.