Getting to see monster Hit Points is a pretty poor reward for gaining Expertise.
1 - It's not all that useful.
2 - Showing us how over-inflated the hit points are was a poor idea.
I suggest that in addition, you make the following changes:
On Expertise gain: Since it's server-wide thing, all characters should receive a X Expertise Feat. This feat is similar to (and stacks with) Ranger Favored Enemy, in that it provides a +1 bonus to attack, damage, AC, and skill checks against those creatures.
On all Character Deeds achieved: That character receives a X Slayer Feat that stacks with he Expertise-based feat.
And yes, it needs to be retroactive, in the form of envelopes in the inventory if necessary.