I think those are some good ideas Duredhel. However, as PFT said, I think "unmodified" rout tests have the advantage of randomness and represent the tension in all warbands involved.
In Mordheim warbands evolve quite quickly, and having a loosing streak can have a considerable impact on money/wyrdstone earnt.
Therefore, you inevitably will have underdogs after a few games. Now if you modified rout tests according to the damage done, you make it even harder for underdogs to win, since they are less likely to inflict as much damage as quickly as the others.
By taking the same rout test for every one, you give weaker warbands the chance of coming out on top, after an encounter.
Besides, routing is really not that bad. In fact, in my experience, half of the time the warband that routs gets away with lesser severe injuries and more money... simply because beating the crap out of the opponent comes at the price of severe bruises for yourself.
I personally would therefore not apply any modifiers, but if it fits the gamestyle of your group, go for it! I think it is a nice little tweak that can potentially make your games more enjoyable (but also more predictable) if all players agree to it.