One Man units (henchmen, champions, sovs) do not usually see a decrease in stats when they are wounded, although there are abilities that tie into this. Said abilities might give an attack bonus once hit points drop below a certain threshold and similar such.
You certainly could code stuff into the existing abilities (say Blood_Amarian, Blood_Tarth, etc.) that do the opposite (-25% to attack when hit points drop below 50% or whatever), and this is something that could be relatively easily done, via the realm of modding if a modder felt the need.
However, Attack Strength versus Defense is a touchy issue, due the way it is calculated, so any minuses can skew things a lot worse than you might think at first blush.
Minuses to accuracy and/or dodge would be less impactful. You'd hit less often/get hit more often, but at least you wouldn't be skewing attack versus defense once a hit is made.
This might drag things out further in combat, however, and with swarm bonuses and such already in play I think champs and sovs are already dying fast enough should they get pinned down against multiple enemy units with good attack values.
