The modifiers for the dark elves' slaves are far too small to make a difference or to be worth pursuing. They hardly have an effect on your factions strength. The diktats in particular are pathetically bad. 1000 gold once every 5 turns? Don't make me laugh. And the global slave bonuses are far too miniscule to make a difference.
The construction effects for having a high number of global slave population should be far higher. Global slave population should also increase income from all buildings factionwide, which would mimic the effect it had in WH2 without the potential for abuse the old system had.
The diktat that gives 1000 gold should be substantially increased, or it should scale up over time. In addition, remove the factionwide cooldown for diktats, and only have the cooldown be localized to the province. This would make the diktats more interactive and interesting, as you would have the option to spend a great deal of slaves immediately for immediate benefits, such as more gold, or hang onto them for the long term benefits to income and construction cost/time. The way dikats are currently implemented is far too restricted and weak.