Having just read some High Elf novels, I can confirm that at the time the High Elves were vaguely aware of some small settlements to the west but not aware of the Slann or LM in Lustria. They didn't start exploring until close to the sundering (when the Dark Elves were created) and even then, only to the Dwarfs further west and the chaos wastes to the north.
As far as lores go, according to the fluff, Slann are masters at shaping the world (creating chasms or mountains, even realigning the continents). Now that sounds like real power to me. No elf can do that. If LM get their own lore, I think it should revolve around that. How awesome would it be to reduce enemy movement and BS with earthquakes, block their movement and line of sight by creating barriers, buff our S or T, and hurtle massive boulders at our foes with a flick of our wrists? Totally awesome. Also, i think HE mages are considered the best in the world because our Slann sleep all the time. Now that Mazdamundi is on the warpath, I think perceptions may change.
I agree with the lore idea, I remember in 5th edition there was a high spell you could cast that moved hills, and there was one that the mage was held aloft the battlefield on a pillar of stone meaning his spells had infinite range and he could see everything
sorry if this has already been mentioned but in the lizardmen book it says "only to the lizardmens stand against the daemons was chaos hold in check long enough for thhe ritual to be performed."