When playing as an Altmer, during this quest you can don Thalmor Robes, and there is a speech check to trick those gullible fools into leaving. What exactly is it?

You tell them Elenwen needs their assistance if I recall right. (I might not, it's been a long time since I've done it)

Trying to talk to the torture chamber thalmor will reveal you as not thalmor though and they'll become hostile.

I know what to do, I've played the game to death. I meant what value does the speech skill need to be to pass the check?