That is correct. You can't Cast powers out of tanks. Put him with the reapers. And doom works well when having lots of low S weapons. You'll wound on 2+ most of the time anyways. Go with diviniation table and take the primaria, guideEq, and hope for missfortune that rerolls sucessfull armoursaves instead. This combo eat anything!
I'm going to go for divination completely on my next farseer - hoping for a 4++ or ignores cover, then getting the primaris out.
With the eldar farseer table you don't want Mind War, Death Mission, Eldritch Storm or Executioner so thats 4 powers out of 7 that are bad.
Going on the divination table, the powers that aren't so good if you're putting your farseer with reapers are forewarning (as they don't get overwatch) and Precognition as your farseer isn't a cc monster.
Prescience, Forewarning, Misfortune, Perfect Timing, Scriers gaze can all be useful, so I'd go divination all the way as thats only 2 or 3 that are bad depending on your build (scriers gaze is situational if you have reserves - as my eldar use 3 FW planes I find it useful)