Let's examine this a bit: What is the purpose of having a health bar? It's to visually represent the % of health that a character currently has and, by visually showing the portion of health that is being removed, representing the % of health that is being removed by DPS. It's not meant to show very close estimates or exact numbers; merely to give a feel. There already is a system to show exact numbers; that is, displaying the exact health of the champion.
Therefore, if your problem is that people don't know health at higher levels display the exact health of a champion around their health bars. It doesn't add toooo much clutter or force a change in readability, it solves the problem of high health display and it let's people calculate their own DPS or soak ability more accurately.