Around 40CP is where you see your biggest gains @15%/25%, after that it's kinda dead - As for Khajit in specific, I'd imagine building flat damage is still better as you don't have a high enough Crit Chance to make it worthwhile, also flat damage scales with crits.

Assuming this is PvE based it's easier for you to test alone:

1. Hit a dummy with current setup (Unbuffed is easier to manage) - Record familiar Crit and Non-Crits with a few abilities.
2. Take out X points from Y(Thaumaturge, Elemental Expert, etc) and test for the same white and yellow numbers
3. Add those removed points to Elfborn and test the same values again
4. Compare and adjust based on what you're finding until you have an answer.

There's obviously more depth to the math than just eyeballing it like that, but you'll get a rough idea.