In a perfect game, strong units have a proportionally large cost. Therefore, the easiest solution to "fix" OP units is to increase their cost. Unfortunately, doing this unit by unit is complex and hard to gain a community-wide consensus. I think we can make a pretty good fix by adjusting point costs by codex.
I suggest Eldar players enjoy the entirety of their codex, guilt-free, by agreeing to pay 25% more in points for all of their units.
I think it would be great if, as a community, we could agree on a scaling factor for each codex. DA and other soft codices pay normal cost. Stronger codices get an overall point tax.