I realize there's probably no such thing as an optimal level here, but in 3.1.2. the power loss seems to be way too fast. In sparsely populated levels, Hornet's Sting loses modifiers while walking from one end to another, despite killing some monsters on the way. A particularly tricky fight against a greater titan (i.e. a few hundred turns without a kill) dropped the weapon almost to its initial stats.
In the top level of the ToEF, I never managed to raise the power of the weapon significantly, because most monsters required around 3 hits, combined with walking back and forth a few squares in between hitting things. At the end of killing every inhabitant in the temple the weapon still had almost its initial stats.
Seems a shame, since I got the same weakish artifact in 3.1.0 and had a tough time deciding between it and a more powerful weapon, because of the speed modifier. In this version, Hornet's Sting and its ilk go straight to the altar.
Suggestions:
1. Make the initial level of the relatively weak blood-drinkers a bit stronger
or
2. Tone down the power loss somewhat