Wouldn't that give a huge amount of piety? Maybe it would be better to simply only give piety for equal-level kills.
The way I've set this up, the more you focus on higher-level enemies the less piety you'll earn.
For instance, a level 5 character with humility requires 30 XP to level up. If you fight equal-level monsters, you need to kill six of them to level up. That means 30 piety under my current setup. However, if you were fighting level 7 monsters (fairly standard procedure for classic GG followers) you'd only kill three before leveling up, gaining only 15 piety. Furthermore, this will reduce the number of equal-or-higher monsters available to you at a later point in time, reducing your piety gain options later.
Due to the rarity of high-level monsters and the fact that piety gain is low at low-levels, there's a very narrow window of opportunity in the level 4-7 range where you can earn real piety. The pickings are pretty slim for level 1-3 and 8-10 characters. Funny how level 1-3 is the best time to enter the religion seeking humility and absolution, and level 8-10 is the best time to enter seeking protection. You'd almost think I'd been pondering this problem for a while