I tried to mod the leveling curve of regular units and heroes, which level at the same speed at the moment. Unfortunately regular units increase their HP with a rate of 3 per level, and therefore have HP-wise a big advantage to heroes. So far i had no luck:

• It seems we can not alter the number of points to distribute per level up via /Core World/CoreChampionLevels.xml, as changes here seem to have no effect ingame. Furthermore, adding static values to HP per level using that file did not work either, so i suspect the data in the file is no longer used.
• Furthermore the required Experience per Level are simply (Level^2)*10, and unfortunately not influenced by <UnitStat_ExpToNextLevel> (which seems to do nothing at the moment). 
• Finally the <LevelMilestones> which were my last idea to change the leveling curve do no longer work, because of the following change that was applied sometimes back in beta:

- Basically made two functions for setting a unit's level: SetLevel and GrowLevel.  SetLevel should be called when initially creating a unit, as it looks in the level milestone info to get the stats that that unit should have. GrowLevel should be used on units that already exist and have a level, and it will grow the unit level by 1 and increase each stat by some percentage of the current value instead of looking in the level milestones again. Therefore, level milestones are useful for setting the stats of units that you create at arbitrary levels, but once a unit is created, they follow a different stat growth path.

So for me it seems that we are struck with regular units that get faster powerfull than heroes, while leveling - because GrowLevel takes the possibility to mod the leveling behaviour out of our hands. The only possible mod to make high level heroes more powerfull, compared to regular troops, seems to change the increase in Strength, Constitution, Dexterity and Essence per point upon level up. 

Has anyone found a way to change the leveling behaviour of troops and heroes that i did not think of?

On a related note: The crown symbol that is shown when a battle has finished is the <UnitStat_Experience> value of the unit and is in no way correlated to the number of xp gained. In fact there is no reason at all why it should be displayed.