Well it wouldn't hurt to make them a LITTLE stronger.
But then again, playing on Death March difficulty I have difficulty with every monster (even below my level) but still can kill monsters 5 - 8 levels above me.
The only difference is the damage they make and since stats do not increase on the 2 highest difficulties it means that basically you will have to get better and better in terms of your own reactions and skills and strategy rather than gaining more "power". Sure, you got skills which can help you, but aside from them there is no stat increase on Death March, which means monsters will stay strong for me. Only thing changing for me is my equipment, bombs, potions and to a small degree the damage I deal with signs or swords.