As for endurance it would be enough if it would just work retroactively.
+3 EN boost would give 6-26 or 6-46 HP (depending on character level and base EN).
-4 EN would reduce HP by 8-48 HP. Seeing mid-fight death from withdrawal would be priceless.
But then, I presume there would be trouble with implementing it, and a static HP change would be simpler and bug-free.
On a side note, HP formula should be altered, so an odd value of EN gives you an extra 1 HP every second level. Same for other stats where possible. Rounding everything to integers on every step sucks ass. Things like the fact that "Very Good" endurance is actually worse than "Good" shouldn't take place.